FirstWatt SIT-3 or Pass Labs XA25 for Avalon Indras?

Hate to be the bearer of bad news but your Avalons have a reputation for being power and current hungry. Either of these Pass designs would leave you wanting, especially at low volumes. I have little doubt that even Nelson himself would tell you something similar. 
 

Pass makes some fine amplification, but so do many others. You’re better off trying a higher power Coda or Ayre amp. The XA25 is rated at 25 WPC for good reason. Certainly, it can output more before it reaches “clipping” at 1%THD, but it clearly sounds strained when doing so. It simply isn’t a high current design, neither is the SIT-3.

The best advice I can give is to cancel the Stereophile and TAS subscriptions. Those pubs survive off of keeping audiophiles on the merri-go-round. Some of the most underwhelming gear I’ve owned and experienced was praised by those rags. 

@ulul 4 Ohms isn't a good load for the SIT amps and is marginal for the XA25. These amps are not high power and really are much better suited for slightly higher impedances since they tend to run warm- lower impedance loads (like 4 Ohms) will make them run warmer.

There are class D amps now that have every bit the quality of sound you seem to be looking for, but no heat (and unsurprisingly) more power. You might want to consider widening your search.

Go to Powermodules.com and get a Belles Amp...I have the Aria Integrated rated at 70 watts @8 ohm...It would drive the A's nicely.

Or call Rich @ Signature sound in Liverpool, NY.

I have both the SIT-3 and the XA-25. The XA-25 is sitting in a box... but it depends on the speaker. I'm using it with my Klipsch speakers. If I owned a set of Harbeth's my hunch is the SIT-3 would be in the box. Unscientifically I'd say the XA 25 with dark speakers and the SIT-3 with breathy horn speakers.