I told you a while ago Hans, this thing is your sound for sure. Just don't use the wrong driver tube 

It wasn't that long ago, unless I'm forgetting a previous time you told me. I picked one up very shortly after our most recent conversation, ha. You have any recommendations for tubes? I'm just using whatever was thrown in with the amp.

Initial tests do make me think it at least has potential to be a bit more tubey sounding than the Val2, and more powerful/dynamic. Val2 can either be a bit too solid state and slightly rough sounding (tubes can change this or shift the rough spots around, so could all be tubes I've tried) or not dynamic sounding enough for me.

I like the mullard cv4003. Also if it has the standard socket watch out with new production tubes with gold prins. They are slightly thicker and mess up your socket for NOS tubes. Mine has a Russian military socket now, much tougher.

I like the mullard cv4003. Also if it has the standard socket watch out with new production tubes with gold prins. They are slightly thicker and mess up your socket for NOS tubes. Mine has a Russian military socket now, much tougher.

Oh! Good to know...thanks for when I try some tube rolling in mine.  And, I love those crazy caps on top.  Like a tube amp version of TruckNutz, but functional!

....Space and resolution: Valhalla. Intimacy and engaging dynamics: Crack/Speedball.

Last week I bought a 6SN7 adapter for Ember from Garage1217, the same adapter works on the Crack for the input tube. The crack looks pretty great with a 6SN7 input tube 

I haven't heard the Valhalla, but this summary description of the crack is right on.  It is dynamic, engaging and punchy and throws a good but not expansive soundstage.  Although more detailed and resolving than the sluggish WA3, it loses out to certain other well designed amps in the resolution department.  This and the slight lack of refinement in the highs did not bother me at all with hd6X0's or dt880's but can be an issue with hd800's, which necessitated a lot of tube rolling to find something I liked.

FWIW - I have not had good experiences with 6sn7 adapters.  Tried twice, one of which came from Lowther Club, and both added audible noise with various 6sn7's as compared to the blessed black background I hear with all of my 12au7's and 12bh7's.  One other user on bottlehead reported the same outcome with three different adapters.  Others report good things /shrug.  Bottlehead, and others, have suggested that 6sn7's may perform better/were intended to run with DC, not AC, on the heater pin, so a wallwart of appropriate spec, wired to that tube socket, might actually be the best solution for those who desire to maximize performance with 6sn7's in the A-slot.

Alternatively, Maxhawk (on HF and bottlehead boards) made a really cool speedball replacement board with a selector switch that changes the bias to run 12au7's, e80cc's or 12bh7's at their ideal operating points.  See: http://bottlehead.com/smf/index.php?topic=5989.msg74147#msg74147  He's a really gracious guy and sent me a board at cost.  Those of you who are interested may want to reach out.

     

Filaments are AC heated?

Yup. On crack they are.  On bh's higher end amps, they get regulated dc.

That's the other key consideration with crack, at the point that the kit, speedball, mods, tubes start to approach $1k (recurring sale price of a mainline), you've run off the rails - which is easy to do given the cost of some of the regularly recommended tubes.  Still love my crack, and don't ever plan to unload it, but mainline is a true flagship amp.  A much more time consuming build also, but so totally worth it.