Well, the band would be taking a break as soon as I get gear to test:evil: All good though, I got a call for a classic rock band...whatever that means. Hope it works out. Anyway, I had the opportunity to try the HB pre with several different amps and power sections and I must say, I'm learning a lot.(I think) Feel free to chime in
Through most solid state heads (with pre's) the Hellborg changes things up a bit and is an upgrade to default tone in regards to clarity. Not "brightness" but real clarity. The only "problem" is that it will still use the head's output circuitry and damping factor. I'm guessing this is why some are underwhelmed when going into the return loop of their current amp. So, the head being used is still controlling the speaker movement.
If you have a really good head like a Thunderfunk, Epifani or a few others I've tried, you could be left with a take it or leave it impression and might as well keep your great amp separate. If you have to constantly switch back and forth and really listen for a difference, I wouldn't call that a big improvement.
Through a Crown XLS power amp, well.....it was loud and clear but the sound was a little thin to my ear. Almost like a dimension of sound was missing or something. I wasn't "feeling" the notes the right way. Mind you, most of my testing was done either in store or at home except for the Thunderfunk and a solid state Ampeg head.
Through the 70's Ampeg V4?

:d I wish they had an emoticon for "orgasmic response"! The sound is huge with all of the clarity and no clangy or brittle sound at all. Most noticeable difference is in the midrange response. It is so "vocal" and woody, with the most awesome and musical (useful) break up when you dig in. Not the usual honk that just gets louder if turned up. Not sure if it's the tubes or the transformers in the V4 but, Damn! The most interesting part for me was the fact that the V4 is only rated at 120W or so, but it easily drives a 500W Berg 2x10 and 2 Aggie 112's strung together. All 8ohm cabs. Actually, the V4 is awesome on it's own since I got a tube and cap job, but with the HB in front of it, I get a real old school tone that I've been looking for and I get it instantly. No need to even turn on the pre's eq and the bass is flat. When I do tweak the eq, it makes me think in terms of more or less "voice" and "growl" instead of midrange and treble. Bass, I now call "hugeness".
Through the Eden WT1250 power amp. For me, this is the perfect blend of old school and modern tone. I wasn't sure how I would feel about my clean preamp signal going through a tone pot, but almost right away I knew it would not be an issue. My speakers never sounded so good. Even the HLF is so clear and smooth now. It used to be a simple blunt instrument to me. I now know the difference between 500w loud and 500w clear while making a room breathe. I don't think there's a tube in there but I did see 2 huge toroidal transformers. It's a dual channel amp with a tone and volume knob for each amp. I found this very useful when mix matching cabs. I can run 1 side at 500W at 4ohms while the other (in stereo) accomodates my 8, 4 or 2 ohm cab(s). It's also bridgeable for 1 massive 1200w channel. The tone knob lets me boost or cut highs or lows to either side independently, depending on the room or cab. It has a good damping factor as well. >100 at 8ohms. Pa amps usually have a much higher damping factor which provides tighter bass when used with PA speakers but if you use bass speakers, the higher damping factor prevents the speaker from moving in the way it was designed and will move less air. You'll get loud but your pants leg won't be flapping.
Remember, some of the old tube heads had adjustable damping, so, if your sealed 8x10 cab sounded muddy, you could switch the damping factor and clean it right up.
The HB pre and Eden power section is MY idea of great tone. Through this or the V4, I have found my ideal tone and truely feel no G.A.S. Can't wait to gig the new set and see how it holds up!
Oh yeah, you don't need to spend a ton of money to get great tone, but in my case, it's clear where the money went. I practice ALL the time now just because I like what I hear.