Nakamichi PA7. The Nak’s are nice old amps and it will drive your Thiel’s no problem. Your Adcom is going to set your bar, meaning the Nak is capable of more revealing preamps. Not sure you’ll find one for $500 but probably closer to budget than finding a Parasound A21 as mentioned which is a very nice and the better amp. The A23 isn’t as good as the A21 and I’m not sure it will drive your Thiel’s that great but with its limited power your Thiel’s may sound a little thin and bright... I do have experience with your 1.6’s. Depending on your taste of music, a Aragon 4004 will open your Thiel’s up no problem. I wouldn’t call the Aragon the most refined amp, but it’s still very good, but a brute with authority. Your Adcom will probably sound really good with the 4004. Your 1.6’s are more efficient than the bigger Thiel’s, but being a 4 ohm I’d find a brute to open them up. If you can find a McCormack DNA1, that would be a nice amp too.