PRIMA LUNA AMPLIFIERS

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I can't speak to the Cayin, which I have heard is made at the same plant, but the unconfirmed rumors are that Cayin uses differently sourced parts  and is not just a PrimaLuna with a different name slapped on it.

Correct, Both are made by Spark Labs,, both look simliar in design,, but that does not at all mean they will present music as equals. "

One distinction to be made here is that Cayin is completely owned by the Chinese government (part of the the CATIC group), while Prima Luna is only manufactured in factories owned by the Chinese government.  So with Prima Luna, you are making less of a contribution to the cause!  Not an issue to most probably but something to keep in mind when making a choice between the two.

Sharing a recent post from the PL Steve Hoffman Forums with some very cool pics of the PL Evo 400 with the KT 150s, as well as an innovative solution for the cage. See page 8. Thanks to @zurv
https://forums.stevehoffman.tv/threads/primaluna-evolution-series.824596/page-8#post-24225408

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.