Most transparent amp you've heard?

Just remember that they have a decreasing impedance with frequency and (to quote soundstage review);
"These speakers can get by with amplifiers of modest power ratings just as long as the amplifier is stable into capacitive low-impedance loads."

In my opinion, the tubes amps on your list that have the best chance of handling the load of your speakers are the CATs (I owned them) - they can drive most anything (the Atma-spheres might work,I own them, but less than ideal IMHO), but you will still proabably be best served with SS (I owned the XA.5s are they are truly great sounding SS amps). Another tube amp that is neutral and would be able to handle the load would be the Music Reference RM9 Special Edition which is able to increase output into dropping impedances and puts out 160-200 watts depending on the tubes you use; if you use the 4 ohm tap the wattage drops a bit.

If you audition the Spectron, also have a listen to other leading class D amps. . . JRDG 312 and Continuum 500 in the Soundings/JRDG/Sumiko Suite, and Mark Levinson No. 53 for example. G.

I forgot VTL Siegfreid. Though I have heard them through Wilson Alexandria at dealer. They sounded wonderful.

G I will check out the Rowland too.

Excellent point Sarcher30, VTL amps are incredibly nice. . . definitely worth listening!