Anyone heard Underwood level1 3910 mod?

I am currently evaluating the 3910 mod. I purchased the Ultimate and sent Parts Connexion my 3910. I have finally let the unit burn in for 200 hours as Wally recommended. So far I am disappointed. The upper bass is overly pronounced and the overall sound isn't open. Vocals never come alive - low level detail seems obscured maybe because of the overly warm upper bass. I am AB-ing the unit with a Benchmark DAC-1. I will say that I don't remember the Benchmark sounding as good as it does now. The mod replaces the clock which Wally claims improves the digital out. I believe that has made a large difference. I have ordered some NOS Mullard 6922 tubes which I'm going to try in the unit next week.

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.

It appears that I have spoken too soon. My disdain for this mod has evaporated. The Mullard tubes with some burn-in time has produced a really terrific CD player. Extremely listenable with great presence. Perhaps a little warmer than the Benchmark, but it doesn't send me out of the room like the Benchmark. Thanks to all for the tube advice - especially Dsremer and Nickturner920. I'm going to call VTS today and order a pair of Siemans tubes. (Although the sweetness of these Mullards is hard to resist).