Hi,
My dual 6BL7 adapters finally arrived, thanks to Mrs. Xuling.
They look like little wing lets, and the Elise is soaring to new heights. It is as if I have a different amp. I'll try to pen my initial impressions.
Firstly, two EL3n are in the driver's seat, powered by an external power supply. The power tubes are 4 x 6BL7GTA from the late 50's and early 60's; a pair of GEs and a Tung Sol and a Raytheon. I have a selection of BL tubes, and I could probably even get together a matching quad, more or less, but these sound good to me, so I chose them for the first trial.
The first impression is volume. With 4 BLs the Elise plays louder than before.
The second impression is power, clarity, punch, detail, detail, detail.
You don't have to strain to hear very subtle nuances in the recording - they jump out at you. The sound stage becomes three dimensional. You simply hear much more of what's going on in the studio or live venue with a much greater clarity than before. The bass goes lower than anything I heard before. Unbelievable instrument separation and dynamics.
I completely concur with what JV wrote above: { I am editing the comments to paraphrase my own impressions}
The details and instrument spacing that the 6BL7 brings out with subtle details and tight articulation in the bass.
Added mid and high freq details , bass goes deeper and is very controlled / articulated.
The larger sense of room/air/space between instruments is also realized.
Listening to the quad BL tubes it also becomes clear to me that the other tubes and tube combinations (even a single pair of BL tubes) mask some of the details in the mid range and bass. Those tubes sound very good, but the quad BL brings more to the table - stuff that you did not know was there suddenly becomes fully revealed.
Glenn wrote that multiple BL tubes increase the drive. When I asked him if I could equate drive with torque to better understand what he meant, he agreed.
It is as if the Elise has so much more reserve power. Imagine driving a small car up a big incline. The engine is struggling to keep up the speed, but it manages. Now, imagine a car with a huge engine going up the same hill - a little tap on the gas pedal effortlessly lets you overtake that little car. The entire driving experience is different in that powerful car.
And the entire listening experience is different with the Elise with quad BL tubes. Old, familiar recordings sound different, with new life and detail and impact. Voices are also very natural.
Now I have to figure out which driver tubes work the best with the quad BL tubes. JV suggested the 6SN7, so that will be next project - will try a favorite pair of RCA and the TS reissue.
I can really recommend trying quad 6BL7 tubes as power tubes. It is not going to break the bank, and there are close to 400 offers on Ebay, some of them very inexpensive. (Mrs X made me a pair of budget adapters without the acrylic plates and without the rotating feature - if you want to go this route you could save some money as well.)
H1, I would really like you to compare your GEC tubes with a quad (or sextuplet) of BL tubes.
With 6A current draw the Elise is just slightly warm...
Amazing......
to all the lucky ones 
