Seeking advice/tips on improving sound quality

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!