Sounds like an interesting choice to have. The second option leaves the preamp at high voltages, but with the reduced headroom of the output stage we are able to drive the output harder and produce more output stage distortion. The first option should keep the character of the amp about the same as the voltage is reduced. I re-purposed the standby switch (who needs a standby switch anyways?) to control whether VVR should apply to the whole amp or just the power amp stage. In theory, it should have much less impact on the tone of the amp than resistive attenuation.
Instead of messing with the output signal, VVR adjusts the high voltage used to power the tubes to control volume. Take the L-Pad out and replace it with a more modern VVR (Variable Voltage Regulator) approach.
FIREFLY PCB MODS
Keeping this section for reference only, but the current version of the amp with new mods is shown below. Many years later I revisited this amp and re-did the mods. These are mods I decided to include in the initial build of the amp without even trying how it sounds stock. I later replaced it with a much more compact 1×10″ cab with Celestion Vintage G10 speaker. As for the speaker, one of the few guitar speakers you could find in Serbia back in 2005 was Celestion Greenback G12M, so I got one of these to make a 1×12″ cabinet to go with the Firefly. Hookup wire is 1000V solid core in different colors, a gift from (HUGE thanks). I experimented with several different brands of 12AX7/ECC83 and 12AU7/ECC81 tubes and tried 12AT7/ECC82 as output tube. For caps I used mostly Orange Drops for nF range, silver mica and ceramics for pF range, filter caps are F+T and Nichicon electrolytics and few caps are nice Solen Fast polyester capacitors. Power transformer was custom made by Trafomatic and output transformer is Hammond 125A. offers many low-wattage amp designs, but I finally decided to go with FireFly, a one or so watt hi-gain amp which offers good cranked tone at bedroom level and is not too complicated to build for a newbie. I purchased good metal film resistors in local store and power transformer is also built here. There aren’t many good audio-grade components here in Serbia so most parts are ordered from Germany, UK or USA. Regardless of that, it’s a great little amp and below is the original build report I wrote back then. I opened it up recently to fix a joint that went bad over time and realized how much progress I made since then regarding soldering, wiring and component placement.