Anyone heard Underwood level1 3910 mod?

Just a suggestion...
I have a APL 3910.
Perhaps your modded 3910 requires a different interconnect and power cord .
Try a silver interconnect like AZ, Kimber Select,Ridgestreet Audio, Purist Dominus, etc.
It may tighten the bass and open the sound.
The Purist Dominus power cord is awesome with my APL 3910.

Kisisam, Up front - I know nothing about your 3910 modded Denon. I'm reacting to your proposed tube change and I'm speculating.

Arn't the Mullards noted for a warmth which might be contra-indicated, especially if you are finding the mod'd Denon overly warm in the upper bass and, presumably because of the 'vocals which never come alive', closed in in the upper mid's and highs and you are finding the Benchmark more appealing. I would have thought a tube like a Seimans or 'Tele' might have been more appropriate. Just a guess/thought.

Newbee, this is from Upscale Audio's website: "Mullard 6922 - Made in Great Britain. In red National Electric boxes. National Electric was the Sovtek of the 60's and 70's: They re-labelled tubes from all over and exported them under their name. These are real Mullards with the dimpled dish getter. Will give you that famous smooth Mullard midrange, with a smidge more energy on top. Great tube if your system is a little relaxed, as they will give you more sparkle in the highs. You may hear a slight "ting!" through your speakers when you move switches, or tap on the preamp. This is normal." Their Siemans are way out of my price range.

What I have found in 6922 tube rolling is that siemens top to bottom are the most detailed, telefunken the most neutral, amprex very nice mids and base, mullards are a warm tube if your system tends to be bright a good option but perhaps not as detailed as the others.
Dennis

Don't trust what vendors say or their prices. The stock tubes on Parts conextion 3910 are mediocre. Try the Mullard it will be an improvement but I agree with Dennis's comments. I run a set of affordable Siemans 6Dj8 from VTS tubes in my 3910 and they are fantastic. And yes I did have the upper bass problem you mention, until I tube rolled. One other thing to consider is that if this is a big source improvement for you do not discount the idea of having to re-position your speakers. You may have placed you speakers to make up for source problems, ie weak bass.