PRIMA LUNA AMPLIFIERS

The switch is for EL34 or KT-88 bias. The KT-120 & KT-150 are a whole different tube than the KT-88. The 120 & 150 use up to 25% more heater current. Unless the amp has a regulated heater supply capable of supplying the increased load, an un-regulated supply may struggle with the extra current. If the amp can supply the current for 150's with an unregulated supply, then the heater voltage for the EL34 and drivers is way too high.

I'm not sure what to tell you from a technical standpoint. But that Primaluna advertise being able to use KT120 and KT150 tubes with their amps and many have success doing so.

The autobias circuit will adjust bias to whatever it is set, not the tube. Different tubes may require a bias change.

Has anyone characterized Hum, Distortion, Power before and after changes?

I own Primaluna Dialogue Premium preamp and HP amp. Built like a tank and not one hiccup in all the time I've had them. 
The A/C offset killer circuit keeps them nice and quite.
I removed all the stock Chinese made tubes for NOS (new old stock) vintage tubes. Mullard, Telefunken, Amperex. Much better in my experience. Heft, tone, detail. They sound warm and lush without sacrificing detail. Better bass and midrange.
Contact Andy Bowman at Vintage Tube Services. He will steer you in the right direction.

I have a set of PL Prologue 6 monos driving a pair of MB Quart Vera VS 1F as my second system and I use Blue JJ EL 34s and occasionally KT88s.  Right now I have PSVane KT 88 Ty II in there.  Both have their benefits and weak spots.  Love the PLs, lots and lots of good stuff no doubt about it and I am not knocking them with the below.

I also own the Carver Crimson 350s as my 1* system driving Carver Amazing Silvers.Why don"t you look at and seriously consider Carver"s new 275.  It's has a 5 yr warranty INC tubes, (nobody else I know of does that), 4 KT 120 power tubes,hand built, with hand wound transformers, made in USA, point to point wiring  75 w/ch and is voiced like the 350s just less power, and $6750 less,  At $2750 is less than most PLs and IMO a much better amp in almost all areas.  I saw a preowed on AM for $2100 + or - . That would be a great bargain, it's a bargain at $2750. I believe Frank (ceo Carver Corp) would probably transfer the warranty if you asked him.  Carver takes care of their customers new or preowned better than any audio corp I've dealt with.  It will drive those ProAcs with ease

BestMark

Why not check out some great American or European made equipment  there’s lots of fantastic choices.