Pre-amp suggestions for Thiel 2.4/Pass Labs XA30.5

As you know the 60 W @ 4 Ohms of the XA30.5 was the Class A output, the XA-30.5 would actually put out another 130 or so Watts in Class B @ 4 ohms for a total of about 190 Watts @ 4 Ohms.
I really am a bit confounded by the ss Macs. Perhaps I'm mistaken, but I thought the use of autoformers prevented doubling down into halve impedance's?

Good link! As far as I can tell from the discussion the ss Mac will only double down if the tap used is higher that the actual speaker demand, and that to preserve the amps stability that is to be avoided. To prevent the amp from being over stressed they use an autoformer with multiple taps, with the idea that one should use the tap that comes closest to the speakers load (I suppose that might be an issue with some speakers with wide impedance swings, the Thiels don't). When used as recommend the amp should not double down and therefore with lower impedance's would have lower dBW output. I could be wrong, but this would appear to be a high powered but not especially robust (in comparison to some other high end ss amps) amplifier design. If I'm mistaken, please anyone enlighten me.

"but I thought the use of autoformers prevented doubling down into halve impedance's?"

That what I thought, the point being that you would get more linear/constant power output throughout the bandwidth without variances caused by impedance swings.

As you know the 60 W @ 4 Ohms of the XA30.5 was the Class A output, the XA-30.5 would actually put out another 130 or so Watts in Class B @ 4 ohms for a total of about 190 Watts @ 4 Ohms.

~~~

Is it the case that many Tube amps don't provide more power as impedance dips?

Yes, many if not most tube amps actually loose power into lower impedance's, and to be fair, most ss amps loose power into higher impedance's (the ss McIntosh, when using the appropriate taps, appears to be an exception to both).