Wow, it's been almost 3 years since I began this thread!
I figured I'd give an update on the questions I asked, and where my system is now.
My first upgrade was a Smart Devices TA-242 power amplifier. The amp is basically a Hafler DH-220 with an improved power supply with a torroidal transformer and some improved components (claimed by Smart). It did everything better than the Adcom GFA-545 mki. Smoother highs, cleaner mids, more seductive vocals, and much better bass. Tighter, deeper, and more output.
My second upgrade soon after was to purchase an Oppo BDP-93. That made another large increase in the SQ of my system compared to streaming Pandora through my TV using its internal DAC. It was a very obvious change for the better, and I was hooked. Plus, I could play CD's again now, and music I had on my NAS drive.
A couple months later I bought a Rythmik Audio F15HP-SE sealed 15" subwoofer and Velodyne SMS-1 subwoofer controller. Wow, who knew how much difference to your music a good subwoofer could make? It was far from the 1-dimensional change I thought it would be. Music was oveall more powerful and energetic. Even up in the midrange it sounded better. And it wasn't just my ears; my wife (who couldn't care less about audio) sat with me one night as I was playing Diana Krall. "Listen to this" I said, and I muted the subwoofer for a few seconds, then brought it back. "Wow, it's like adding a whole other dimension to the music!". The SMS-1 lets me tune the sub to my room, as well as allowing easy adjustment of the sub level via remote.
A few months later I bought a BAT (Balanced Audio Technology) VK-31se preamp. Now that's what I'm talking about! I was concerned tubes wouldn't have the low end I wanted for my electronic music listening. Boy, was I wrong. The BAT has bass in spades. Deep, powerful, tight, and controlled. And detail; who knew baselines were anything other than fuzzy booms of different frequencies. For the first time I heard what I had read people talking about; "texture" in bass. And the black background. It's real, lol. The space, and air, and separation of sounds and instruments. Wow. The leading edges of sounds (percussion, particularly) were so much more "now", crisp, and powerful, with drive and energy I'd never heard before. Probably doesn't need to be said, but it KILLED the Carver HT receiver I was using as a preamp. Not just in a different league, on a different planet. This was the point when things really began to come together. Listening to music and a new track begins and you just smile unconsciously. Awesome. I can't say enough good things about this preamp, and it's not even the current model, or top of the line from BAT. At this point I don't know if I want to listen to anything better, as I like this so much and feel I could live with the 31se happily for many years. I actually started to feel sad that I had missed out on 20 years of "good" sound. If only someone in my college years had sat me down and made me listen to a really good system I may have gone down this road long ago. If it was loud, I was happy.
Not quite a year down the road and I began to wonder how much a better amplifier would improve my system, so I began to look for an amp. I liked the VK-31se so much that I figured I should try a BAT amp. I began looking for a VK-200, but then knew I'd wonder what a real tube midrange sounded like, so started looking for a VK-60. I emailed Victor (of BAT) and he suggested I look for a VK-55se, if I could stretch my budget that far. I found one here on Audiogon not long after for a reasonable price and made a deal with the seller. I picked up some Blue Jeans balanced cables to connect the 31se to the 55se and had my first listen. I have to admit that I wasn't overwhelmed at first listen. But, the kids had been terrible all day, the wife yelling at me, and the amp only had a few minutes to warm up. The next day I was in a much better mood and less stressed, and the amp and preamp were on all day before I did any real listening. Wow, it was a revelation! Not sure how I couldn't hear what I was hearing now the day before. Now the stereo image is totally free of the speakers (you can't even tell where they are with your ears), and the soundstage is so wide open and clear. I'm in love with female vocals. Ambient electronic music (The Orb, Massive Attack, etc) is even more open and spacey. All the sounds are distinct and easy to tell apart. The only thing that I feel could use improvement is soundstage depth. But, I think that is a factor of the BDP-93 and not the BAT pieces.
Which leads me to my current quest, to improve the SQ of streaming music I listen to. Currently a large portion of my listening is Pandora streamed at 128K through the BDP-93 (Blasphemy, I know. But, I don't have an extensive music collection, and it exposes me to many artists I would never otherwise know exist). I want to stream MOG at 320K, but I need an external DAC for that. I've done a ton of research over the last year or so, and am looking at the Wyred 4 Sound DAC-1 or DAC-2. I'll get an Apple TV also, and that way I can stream music from my MacBook and download some hi-res tracks to see what I've been missing.
So, back to my original thread. I'd say the preamp made the largest increase in SQ in my system. It's a lot closer with the sub and the amp. With lower quality streamed music, I'd almost have to say the sub gave me a larger SQ increase. If you focus more on jazz and female vocals only, it might have to swing in favor of the amp though.