I can soundly recommend the move to 150s in the HP...I went from 88s to 120s and noticed the same changes. I didn't get so much of the fatigue but did hear the harshness / glare at higher volumes. Great sound but it was there...but not enough to go back to the 88s or 34s.
Then I wanted "more" and after much good advice and reading, popped the $800 or so for 8 150s. Everything I could find concern with on the 120s were removed...from the second I popped them in. With burn-in they got even smoother. Quite simply...I would never go back...I really love this amp with 150s.
The power has always been overkill for my Zu speakers, but they respond so well to high-energy passages that I think it improves the already impressive dynamics even further.
I have the integrated and it's superb, but I didn't realize how phenomenal this amp was until I put a few really good pre-amps in front, and the further improvements were astounding. Running through the home theater bypass inputs, you still get the phenomenal headphone amp too. I own a First Watt SIT-2, which is also a phenomenal amp, but the HP with 150s is just slightly better or at least as good in almost every way. I could only give the slightest nod to micro dynamics to the SIT-2...but this is slight at best. The HP (with 150s) has even more punch and feels even more like live music...which I know is insanely high praise given how good the SIT-2 is. However, the HP is only that good with 150s, in my opinion.
There are two downsides to 150s in this amp...the cage doesn't fit on any longer (if that matters), and on a personal level, I do not think they are a sexy-looking tube, so you have to look at them even more with the cage off. But sound wins over vanity!
I haven't read a post where someone regretted the upgrade for any amp that can take them. I doubt you will prefer the 120 or 88 sound over them.
