To confirm this tone-sucking behavior, I rigged up the following very simple signal chain:
Fender Strat (bridge pickup) --> A/B Looper Switch (Envelope Filter in A loop) -->Blues Junior amp
First I ran the guitar straight through the looper switch into the amp--tons of sparkly treble, as expected. Next, with the EF in bypass, I switched the A loop into the signal path--MONSTER TONE SUCKAGE, huge treble loss. So, as I suspected, my own EF always had this same issue, but it was just masked by running behind another pedal which was not true bypass and had an input buffer. To verify that, I then put a Boss BD-2 Blues Driver (buffered, non-TB) in the A loop in front of the EF. Suckage gone, treble restored.
I also tried the pedal with the power plugged in and disconnected. While there was somewhat less treble loss with it unplugged, it was still sucking major league tone. So I didn't see the big difference between powered/unpowered that AbeBiro reported above.
The following fix was suggested by "Mr. BYOC", Keith Vonderhulls, and my understanding is that the EF instructions are currently being updated to include it. I have just installed this fix on my own EF and have confirmed it to be 100% effective in eliminating the "tone sucking" problem:
1) Cut the trace on the back of the PCB where shown in YELLOW
2) Solder in a jumper wire (shown in fuchsia) between the two indicated eyelets on the PCB just above the footswitch. These are labeled "7" and "8" on the silkscreened side of the board.
That's all there is to it--carry out these two steps and enjoy your Envelope Filter sans tone suckage!
Cause of the problem: Through a design error in the layout of the traces on the PCB, the output side of the effect circuit was ALWAYS connected to the pedal output, regardless of the footswitch setting. In bypass mode, this permanent connection to the output side of the effect "loaded" the guitar signal and disproportionately affected the treble frequencies, giving that characteristic dull, lifeless sound.