Preamp Suggestions??

Hmmm...could tubes be in your future?
Personally, I'd lose the amp first. Recently I picked up a class A rated amp and that may be a first step for you. It's a PSE Studio IV. 100 wpc, warm, smooth and tons of detail. Find this or a sumo, threshold, Yamaha M series or many others for $350 to $500.

I agree with Elevick. A preamp isn't going to do much without changing your amp. Nothing can tame that Adcom GFA-555. It has a lot of power, but it's like a bull in a china shop. A tube amp, on the other hand, will offer a midrange warmth that sounds more like the musicians are in your room. It will also smooth things out and make your system more listenable and less fatiguing.

Adding a tube preamp to a low end solid state amp won't achieve the same results. I've tried. Many people say it does, but it's wishful thinking -- especially with an Adcom.

Your KEF 103s are 92 dB efficient, so you don't need a lot of power. 50 watts is plenty. If I were in your shoes, I'd sell the Adcom gear and put the money toward a tube integrated. IMO, an integrated gives you a better bang for the buck in this price range. You also eliminate a set of interconnects and a power cable. Stay away from the tube hybrids. Here are some suggestions:

Jolida JD-302B
Jolida JD-502B
Primaluna Prologue One
Cayin A-50T
Antique Sound Lab

Onix SP3 Melody. Beautiful integrated amplifier.
I saw at least 3 in your price range. There was also a brand new model from certify dealer for $500!!!!
Here is the link to Srajan Ebaen's review on *6moons*

http://6moons.com/audioreviews/onix/onix.html

Or if you want something just a bit different sonically and visually , check out Promitheus Audio TVC SE for less then your budget but it's the passive preamp only.

Here is some information:
http://www.enjoythemusic.com/Magazine/equipment/0407/promitheus_audio_tvc_se.htm

And maybe this one too. It has headphone amp and USB for connecting your computer. It might have enough muscle to handle your speakers but read the review first.

Glow Amp One (in your budget. Integrated)
http://www.6moons.com/audioreviews/glow/one.html

08-19-08: Mrjstark
Onix SP3 Melody. Beautiful integrated amplifier.

I see one in the classifieds now for dirt cheap. I'd look into it. Check out the 6Moons review. Here's the distributer's website with a tiny bit of info on the SP3: http://www.melodyhifi.com/SP3.html

As one poster mentioned, going to a tube pre will help, but the Adcom 555 needs to be cleaned up to gain some real benefit.
The 555 is fairly easy to work on. Unscrew the top cover.

You'll see teh blue power supply caps. Add some film and foil bypass caps to each of the main filter caps. Will make a large improvement in the sound quality.

then get a tube pre amp and you should have a system with will sound significantly better.

best of luck