The issue I had mentioned earlier regarding the ML 3xx series has been well documented on the web. Harmon no longer offers the .5 upgrade. I had asked when I sent my amp back for fix 2 years ago.
I think system synergy will play a big part. So you need to think about which preamp you will be using. I am using a GNSC modded ARC LS-25 Mk I.
I had a chance to hear the Pass X350 next to the X350.5 at a friend's house. Huge difference. All present agreed that the non .5 version sounded more like the typical SS amp, cold and sterile. The .5 amp has that midrange warmth that earlier Pass Aleph amps were well known for.
FrankC