I have not tried any of the EL84 based amps in Mesa's product offering. I much prefer the large bottle amps as there are always variants of the tubes available from the EL34 and associated variants like the 6CA7, KT77. Same with the 6L6GC variety of different designs. As for the EL84 it is a limited amount of resources, the alternates are 7189 and 6BQ5 (also classified as EL84). When I look at most places that sell tubes, there seems to be a wider offering that it was 10 years ago when I was looking for tube alternates for a Carvin V3MC. I gave up on the EL34, still not a bad amp category to be in.
As for the spongy/Variac power mode, most will not explore it. Spongy mode gives the amp a change in its voice. A bit more scooped. A bit closer to the MWDR sound in some ways. Too bad the BAD does not have tube rectifiers as that adds a bit more slump and brings out more harmonic content. That inspired me to explore spongy power with tube tracking with the MWDR and Roadster. Totally different amps than the BAD. Does it bring out more cool stuff? Depends on what you mean by cool. I sort of like it but most of the time the BAD is at full power, 100W. If I run both of mine, I may explore the 50W or 25W power modes.
I feel Mesa should have done the full Dyna-Watt power section like the TA-30. That would probably encroach on the 50W model as the 4 EL84 tubes could be equivalent to 50W depending on the OT and overall design. That would at least provide 40W/30W/15W. It could have been modeled after the Mark V:35 for the 35/25W/10W. Not really sure why Mesa decided to set it up as a 100W, 50W-25W tier.
I would not be surprised if Mesa decides to change their portfolio of offerings. From the grapevine, most seem to want the 50W as it is more aggressive. Looking at the images of the 50W, those transformers almost like they are the same as the 100W. If they are the same transformers, the 50W would be similar to the 100W running at half power. That gives you a bit more power tube saturation. Sort of like the RA100 when you run it at 50W. Unlike the TC100 vs the TC50, those are different, and many complain about the TC50 lacking in low end but there is plenty of that with the 100W version. I thought the TC50 was sufficient but had a dryer tight tone compared to the 100W version. I still have both of them.
How much does the BAD25 sound different or the same? Only way to find out is to go to a dealer and play them side by side. I did find an Ola Englund video on the BAD25 and that amp sounds really close to the 100W version. Still the EL84 tubes will provide a different characteristic sound compared to the stock EL34 tubes in the larger version. There is a video comparing the BAD25 to the BAD100. Interesting they are very close in the recording. There will be some slight differences in the room though. No demonstration on the spongy mode.
Again, back to what would answer the question directly is to sit down with one at a Mesa dealer of your choice (assuming they have inventory to try out) and hear for yourself what the varaic power mode gives you. It brings the crunch mode closer to the MWDR sound (scooped effect). Still, it will not give you that sub-harmonic depth of the MWDR.
