Honest Experience on Effects of Subwoofer Please

The way I look at it (YMMV) is if you can swing a quality set of speakers that plays into the 20hz region you are way ahead of the game. A speaker that has good material and integrates well will run you near $20k new (YMMV). At this point you have two boxes with excellent bass down to say 24-30hz. From here you can use a sub to cover the rest. I would never want to play a sub to 35 or 50hz. 

You will get lots of opinions and mine is free and you get what you pay for. 

From personal experience, deeper bass rarely loads seamlessly in the same plain as the mid/highs, and will likely interfere with them due to the greater vibration that they create

Some 10 years ago I bought SVS subs as they were inexpensive compared to audio dedicated subs. I finally sold them last year as I could not get them to blend  as their plate amp choices volume control was too sensitive, meaning even barrel cracking it, the bass would overpower the room. So, it seems high quality subs are essential, and SVS may well make them now, but not then.

hth 

I admit I did not read everyone else's answer, so apologize if I am repeating something.

First, what are your main speakers?  Some main speakers need a sub more than others.

A really good sub will be better at the lowest octave than pretty much any speaker.  This is not subtle, it is very, very obvious.

It's a matter of personal taste whether the sub is integrated well enough with your other speakers for your liking.

Benefits to the midbass are real IF main speakers are being CROSSED OVER at a higher frequency, that is to say if BASS frequencies are being diverted from the main speakers - they will be moving with smaller excursion so that should give a cleaner midrange, greater power handling.

If main speakers are still playing the whole range then benefits to mids would not be as obvious. Maybe could still occur somewhat if your need to turn the volume knob up changes with the introduction of a Sub.

"There is no replacement for displacement" (zero fidelity?)   If you need loud music in a large room, played with authority and accuracy, you need large woofers.   A more modern possibilty would be the use of small woofers combined with a ton of power and digital processing as some of the modern subwoofers just coming out recently.

There is a YouTube channel called Nemo Propaganda Reviews.  This dude knows more about subwoofers than anyone on earth!

My experience has been that subs are good for Mid-Fi and Home Theatre (not so much for HQ 2-channel).

Inevidently, once I got the a sub to integrate sound pressure level wise, with my speakers (current rotation is Quad 63, ML ESL and ET LFT VIII), in my dedicated listening room, it screws up the Detail, Timing, Sound Stage or Image. Over many years I tried Electronic Cross-Overs (the closest to good), DSP and passive circuits... the experiment is now (as of last month) permanently suspended.

To each his own...YMMV.... 

I chased bass nirvana only to realize that what I was really missing was great midrange performance. I ended up selling my Definitive Technology ST-L speakers (towers with powered subs) and a single Martin Logan 1100x Sub.

I listen to a wide range of music: Mostly jazz, female vocalists, some live rock and blues, some country, even some electronic dance music.

I now have Harbeth 30.2 XDs with 2 REL T/9x and I'm really happy. I use the speaker level connections from my tube amp but I also use the Low level sub-cable connection from my A/V processor for TV/Movies so I get the benefit of stereo bass for my analog system and low level effects for my 5.1 surround sound system.   

A1: In my stereo pair set up the effect is subtle and depends on the recording. They add power and depth, a richness that is something you can feel. I have them located very close to the main speakers, phase is 0, crossover 38Hz (estimated, 5 clicks) and gain is 10 clicks (less then 1/3). Of course your own room and speakers will dictate these settings. I probably adjusted these 100 times before I got to the point where I didn't feel the need to fiddle with them anymore because they "disappeared". 

A2: They fill in the bottom end. The kick, dynamics etc. come from the Harbeth's amazing mid-range. They cut off at 50Hz but that does not take into consideration the room which in my case is large. 

*Most of the music is above 40Hz

*You need a stereo pair, otherwise you are summing 2 discrete channels of low frequency signal into one speaker - seems really obvious when you think about it but I was doing it wrong for a few years before a good hi-fi shop owner showed me the way forward

*I may not have bought the RELs if I was willing to buy a more powerful amp (mine is 40w pc) and large, full range speakers but in comparison, what I ended up with was a bargain and I no longer feel like I'm missing something.