Pass Labs xa - .5 amps

Shadorne, you nailed it. Basically, the tube amps produce varying decibel levels across the frequency band, with the higher frequencies (and the higher impedances) generally having higher decibel levels relative to the lower frequencies (and the lower impedances).

There are several Sonata III owners who happily use tube amps, so it seems we are not all equally sensitive to tonal imbalance.

Tvad, I was doing a search on the silverlines, (I've always wanted to hear them) and I saw several mentions where Alan Yun preferred tube amps. It might have been for a different model. Regardless, it sounds like you've sorted it out to your liking, and after all said and done that's the important part.
Just because the Pass gear didn't work for us doesn't mean it won't for somebody else. My new mantra: Listen for yourselves, people!

06-04-09: Chashas1
Tvad, I was doing a search on the silverlines, (I've always wanted to hear them) and I saw several mentions where Alan Yun preferred tube amps.

I attended two CES shows (2004 and 2005), and one LA audio show (2005, I think...) at which Alan demonstrated his speakers with a Pass Labs XA-600 amplifier. He also had a Silverline 300B tube amp available if anyone wanted to hear his speakers driven by tubes.

During discussions with Alan, he told me he voices his speakers with Pass Labs, and it was during one of those conversations that he recommended Pass Labs as an optimal match for the Sonata III speakers when I mentioned my concern about a perceived brightness in the speakers. He also mentioned Belles amps as a less expensive alternative.

...as has been mentioned in other threads, since the topic of Silverline speakers and tubes vs. solid state has been thoroughly discussed, different Silverline models work better with tubes than other models due to flatter impedance curves. The Bolero model is one example.

So, one has to consider specific Silverline models rather than generalize about the brand.

In the example above we are talking like a maximum of 4 db difference this is not dissimilar from what some people might do with a tone control to adjust for room acoustics and preferences.

I found a good explanation here

Please note that if you have a speaker with a fairly flat frequency response (varies between 6 to 12 ohms) then differences in presentation from a low output impedance amp versus a higher output impedance amp will become considerably less.

It is not that a particular amp is right or wrong or a particular speaker is right or wrong => it is just important to consider the two (depending on your amp you might steer clear of certain speakers or depending on your speaker you might steer clear of certain amps. It is all about synergy.