Aragon 4004mkII OR Adcom GFA 555 II

The Aragon is definately the better of the two; actually as bigger powetred amps go ... a great amp at used prices. More resolution, finesse, and .... more transparent for the buck.

The Aragon will be smoother but too laid back and quite boring in my opinion. Chose a McCormack DNA 1 when I auditoned both of these years ago. For your Thiels, I would look for used Classe at the price range you mentioned. I just found the Aragons too slow and univolving.

i've owned both the adcom and the argagon and think the aragon is overall the better of the two, and a good match with the thiels. the mccormack would be even better though. it's dynamic without being bright, very warm and full midrange.

I have owned the GFA 555II and the 2004mkII. I think the Aragon is the superior amp across the board. However, it does not necessarily mean it will be a good match with your speakers. I have not heard the Theil speakers, but from what I understand, they can sound forward. This is also the case with the Aragon amp. I think the 8000 series amps are much better in the mids and highs and may well be a better match. The 8002 is a seriously underrated 125 wpc and maintains the improvements of the 8008. I have owned both series of amps and prefer the 8000 series for the reason I mention. The 8002 does not come up for sale that often, but can be had for about $550 to $600. Again, make sure that what some consider a strength of the amp does not become a liability because of bad synergy. Hope this helps. BTW, I've never found any Aragon amp to be dull or uninvolving. I think they are one of the true bargains in audio.

Totally agree with Csmgolf.
The 8002 is underrated, for whatever reason. I used this amp with my THiel 2.2s and Dynaudio 1.8 MKII's. It drove them very well and maintained a nice coherent sound at very high levels in my mid-sized room. This series is superior to the 2004,4004. Snatch one up if you can.
Please do not make the mistake of pairing Adcom amps with any Thiel product. Again, I second Csmgolf.