There are two topographies in the Pass Labs lineup. The “X” series are class AB amps that are biased into Class A operation for the first 5-10 watts or so of power, after which they operate in class AB mode. These draw less wattage at idle and don’t put out heat like the “XA” series amps that joesphlg refers to. The X-xxx.8 amps superseded the X-xxx.5 amps.
The “XA” amps operate in Class A mode up to their rated power, so they draw their rated wattage even at idle. They do get toasty. As with the “X” series amps, there is an older XA-xxx.5 series and the current XA-xxx.8 series.
According to website specs, the X350.8 draws about three times as much wattage at idle than the BHK 250 (~500 watts vs 175), so I’d expect it to run warmer.
I worked my way up the Pass food chain over a period of years by buying and trading in used or demo equipment through Reno Hi-Fi (Mark will buy back / trade in any used Pass gear and makes a market for used Pass equipment). The XA .5’s sounded a bit more dynamic, with punchier bass. The XA .8’s were less so, but had greater transparency throughout the soundstage. So there are differences. Either way, they are really solid (and heavy) and sound gorgeous. Nelson could have designated the series as “OK” as in “OverKIll” or “BA” as in “BoatAnchor”
I have heard the BHK’s at the RMAF show in Denver, and in PS Audio’s Sound Room 1. They are no lightweights. either. Knowing how PS Audio and Pass voice their equipment, I don’t think you could go wrong either way.
Nelson Pass has an interesting white paper that touches on distortion and amplifier voicing here, if you’re interested: https://www.passlabs.com/press/audio-distortion-and-feedback
Long story short" “Anecdotally, it appears that preferences break out roughly into a third of customers liking 2nd harmonic types, a third liking 3rd harmonic, and the remainder liking neither or both. Customers have also been known to change their mind over a period of time.”