Seeking advice/tips on improving sound quality

Greetings fellow audiophiles,
Lately I have gravitated towards streaming music from my iMac to a Audioengine B1 bluetooth receiver just for the sake of convenience and the ability to play the same music in my main listening room as well as the patio/backyard (which is extremely convenient when we have large gatherings) The primary system comprises of Mcintosh C27 preamp + Mccormack DNA .5 + Vandersteen 2CE and Vandersteen 2WQ. It's a modest system and works very well. 90% of my audio files on the IMac have been ripped from original cds to Apple lossless format. Although this setup sounds very good, I was curious to know what else can be added to this chain that would bring about significant improvements to the sound quality, i have no experience with any software packages that claim to enhance the sound quality and have occassionally used an Arcam rDAC which has not sounded any better than the dac that's in the Audioengine. 
#1 improvement will be to upgrade the Vandersteens to 3's or higher and work on placement & room treatments (or patio treatments)

#2 is to got back to those original CDs and find better sounding ones - better mastering - esp. important if they are older releases

if the bluetooth is older see if you can move to WiFi - did the Arcam unit use WiFi?

Mcintosh has a "house sound" and many feel it is rather dark - so...

Of course going up the Vandy line will yield better reproduction, but I think the OP wanted to ugrade sound quality for his networked system.
I, too, have been working on this. 
Just last week I bought Fidelizer and have been wowed by the difference in my system. Since you are using a Mac, I am sure there are other programs that do the same thing (optimize OS for music reproduction).
A quick Google brought up something called Museeks. I don't run Mac, so I can't say if it is good or not, but maybe worth a try.
Bluetooth is newer and am using AptX protocol. Vandersteen 3 might be too big for the room, besides the 2CEs are the signature ii editions (3 yrs old)... so not sure if all that hassle of swapping speakers will be justified in sound improvement. The Arcam is the non wifi one. Guess, experimenting with another preamp might not be a bad idea if there are any recommendations.

Thanks Bob. Someone at work mentioned JRivers software. Swears by it. Apparently it has sound processing/optimization enhancements that are meant for audiophiles to tinker with. They have a 30 day trial and might give it a shot.

1-Do you have the Vandersteen stands filled?
2-Try a really nice dac if you can borrow one?
3-Cable upgrades?  I just upgraded my power cord (nordost) and it actually made a nice difference.
4-AC conditioner?
5-Buy a turntable?

I like your system.
elevick, thanks for the tips. Stands are not filled. I will try it out. As far as cables are concerned, I have Kimber PBJs all the way through for interconnects and Schmitt Audio speaker/subwoofer cables. The Mccormack has a Shunyata Venom power cable. I wish I could use a custom power cable for the Macintosh but unfortunately that's not an option. Speakers are bi-wired. I have a 8 year old Monster power conditioner. It used to be their top of their line back then so am assuming it is good enough. I am itching to go on the turn table route but it would take me a lot of money and time to build my connection again on vinyl. Maybe I might find out if any local shops can let me experiment with a DAC on 'try before you buy basis'...I am also contemplating on what can be done with room treatment that doesn't start a war in the house over aesthetics if you know what i mean....
I would look at getting a nice dedicated DAC to connect to your pre-amp and then connect the IMAC to the DAC (USB is fine).  The better the DAC, the better sound your going to get.  Honestly, I don't think upgrading the software on your Mac is going help that much.  What's your budget to improve the sound from the computer?  
Just to share my story. I started off using a Mac mini to an Ayre Codex DAC, but I wasn’t happy with the SQ, too much USB jitter. I was thinking about adding all kinds of tweaks like linear power to the Mac and other tweaks (USB isolators, Sonore MicroRendu) and then I realized that with all these tweaks, it would have ended up costing the same as buying a audiophile music server (without all the work), so I ended up buying an Aurender N100H and now the music the Aurender + Ayre Codex can produce is very very good. No jitter. One option is to ditch your iMac and put your music on an Aurender or Melco N1A like audiophile level music server and connect it to a good sounding DAC. It’s a lot of work and money to get computers to sound good but I think your best bang for buck is an upgraded DAC.
Nycjlee, thanks for sharing your experience. I have heard about the Aurender but haven't seen or heard one. Will find out if there are any dealers here in the San Diego area who carry it.

Bluesound Node 2 plus a Tidal HiFi subscription. Or depending on your budget, go with a new Bryston BDP-2 or a used BDP-1 if you don't care about MQA.
If the goal is to improve the sound from your main system at the source to your preamp I suggest a playback program such as mentioned here and purchasing a used DAC.

I use Pure Music program and PM allows a 15 day trial period. There are many others. I would purchase the DAC prior to giving any program a try. 
I guess based on everyone's feedback the lowest common denominator is a DAC. For some odd reason my Arcam RDac doesn't jive with the Macintosh pre amp and when I turn up the volume all the way to the max I get a sound level that is moderately acceptable. Hence I am a little reluctant to purchase a DAC only to find that I am still experiencing the same issues. I will take my C27 to local dealers to find a day that can actually work well with it.

Do you have the system on a dedicated line, and if not, have you ever considered doing a dedicated line with upgraded outlet?

The weak link is your source.  There are various options as mentioned CDP, DAC, build your own server which will sound better than the MAC, etc.  Best option is a really good DAC IMO this will give you the biggest improvement.  You really don't describe what you are searching for so it is hard to know what to advise you on.  It is best to mention what music you listen to and where is lacks what you would like to change.

Happy Listening. 
Bigkidz, you hit it on the nail. Unfortunately I don't know what I am looking for myself outside of just exploring what can help improve the current sound. I predominantly listen to classic rock and roll (mostly Floyd, Led Zep, Who) and then jazz (Dave Brubeck, Miles, Monk etc) and then a few odd categories in between at times. My desire to improve the sound came up when I noticed differences between CDs that I was playing to the corresponding recording streamed from the iMac. The difference was primarily in the separation of instruments which sounded very good irrespective of the source on the jazz recordings but not so much on the rock and roll ones. Hence I started questioning whether the recordings themselves were the culprit or it was something in the setup that needed a boost...Looks like all roads are currently leading to a good DAC so I will explore that option. I guess my final goal is to have the ability to get CD like sound but through the convenience of a music organizer like iTunes etc. so that I can listen to various different songs/artists without loading/unloading cds. 
Are you using a macintosh CD player?  It sounds like you definitely have a source problem. If a CD player sounds good through your pre-amp, I don't think it's your pre-amp or anything down stream. Let us know what kind of budget your thinking about and I'm sure people can recommend some good DACs in that range.  
Uditnait, several comments in response to your last 2 posts:

Many jazz recording are miked and processed such that the imaging within the soundstage is more focused over that of many rock productions. 

On your MAC make sure you have volume set to max, reset iTunes to rip CDs as AIFF files, use the error correction option, and consider another playback program (as aforementioned). 

Good idea to take your preamp with you to audition DACs. Are you willing to purchase a used DAC? There are many older ones which are very good, built with robust power supply and excellent analog output stage which can be had for a fraction of the original price. One caveat, with those over 5 years old you might need a USB/SPDIF converter as USB implementation has improved.  Older ones are isochronous vs. asynchronous. Consider your input needs and future use options (DSD, hi-rez) prior to selection of a DAC.

I am very happy with my MAC Mini, Bryston BDA-1 with USB converter, playing AIFF files over Pure Music in my system. I find redbook CD quality quite satisfying and far less expensive for an extensive library. I occasionally like to spin CDs so I require two coax inputs with a DAC, however have not current need for DSD.

Good luck in your search!