i think a lower powered 6V6 hiwatt would be great

I thought the point of a Hiwatt was huge amounts of clean headroom?

my 80's Hiwatt has tons of clean headroom but if you turn the preamp gain up it screams, JCM800 gain territory. It's only a 50 watt and this thing is LOUD!

I think Nik needs to make a "Lucky 7" type amp. About 7 watts with a large amount of clean headroom available but with alot of gain available too

Because the Hiwatt OD sound requires both preamp and power tubes working.  Adding a Master Volume to turn down will lose all the power tube crunch and just give you buzzy, saturate preamp OD. 

Master Volume amps are designed around their preamp tone, vintage-style non-MV amps are all based around the sound of being cranked, which is power tube overdrive.  You'll need either an attenuator or voltage regulation to drop that volume.

General rule of thumb, you half the voltage for the dirty amp.  If you have a 100-watt clean amp, you want to pair it with a 50-watt dirty amp if running a clean/dirty setup with balanced output.  That's why amps like the Carr and Toneking turn off two power tubes in OD mode. 

Im goin to have to disagree with you man. yes part of the sound comes from the power tubes being overdriven but my Hiwatt doesnt sound thin or buzzy with the preamp gain all the way up and the master at tolerable levels

Actually, on the Hiwatt circuit I have in my HeyWhat, I think it is possible to include separate master volumes without influencing the tone coming from the preamp much. That way you have more possibilities with the amp. The difference would be that when an original Hiwatt circuit is cranked, the two channels are combined just before an overdriven preamp tube. If I mod mine, the two signals would be combined before the next preamp triode, which is after the tone stack, and the signal is therefore a bit weaker there. Also, that tube is differently configured. So there could be some noticeable difference. I would like to try it some time.

I assumed many Master volume amps are designed around both preamp and poweramp distortion/tone, not just preamp. Also, I would expect that many non master volume amps were not designed to distort at all, since there are many electrified musical genres where distortion is not considered to be good.

I expect a fantastic tone from my HeyWhat, completely independent of what the original Hiwatt tone is. Actually, I got this model because it is a fantastic bass amp at the same time (with some small mods).

Attenuator is a good idea. That way you can also have output transformer distortion. I think the tone of preamp distortion can vary a lot, depending on how much careful though has gone into the design. If you don't design for a great preamp distortion tone, it will not be there in the amp.

Good point about the wattages. I have a couple of smaller guitar amps which fit your scheme. I can use my HeyWhat 103 with two power tubes pulled for clean (or no tubes pulled), together with my Mesa Express 5:25 for dirty. It is about half the wattage. Or my 10-20W (dep. on which tube I use in it) DIY AX84 SEL amp for dirty. And the top of the cake (as we say in Norway): I can have all four tubes in the HeyWhat, and use it as a bass amp!