Wow. I hope you make beautiful noise with that amazing amplifier. I was a roadie in the 80s for a band that had a hardwood IIB combo. It was heavy artillery. 🔊🎸🎶🎵😁
EVM12l is rated as low as 80hz.
EVM15l is rated as low as 60hz.
EVM15l were sold in loaded Mesa bass cabinets.
A bass low e is 41hz which is theoretically out of range of the evm15l bass speaker.
My Mesa 2x15 with EVM15l speakers works fine at stage volume. I have never used my bass through...
From reading your post, it would seem you need to run in front of your preamps or you need a "buffer" after it if you use just before the power amp. You have to give the power amp this signal it needs to function properly in the rig.
You can get a lot of opinions. Since you cannot blow...
You can just use your effects through the effects loop if parallel or pick up a suhr mini mix or similar item to create a parallel effects loop. Less $ than a power amp.
Not a good idea. This would cause phase problems. You can even have those issues with separate cabinets.
You could run effects direct to board and they can mix in with your dry cab from there.
There is a member here GTS that does exceptional work. You should do a search.
When you send a head case to get retolexed you remove the amp. This way they will not damage it and if there is shipping the weight is significantly less.
Good luck.
Either run in parallel like you suggested or . .
Run guitar to amp and then into a palmer and/or mike a dry cab. Run the palmer/mike signal to a rack with effects and send to wet cab(s). A wet/dry system will give you what you want. But will also give you more gear to haul if gigging...
My understanding is no and that it will burn through the 6v6 tubes quicker and could also damage the amp. Put 6L6 tubes in that amp.
Keep in mind someone may have modified the amp to run 6v6 tubes. I suggest posting good pictures of the naughty bits so the more technical members can tell you...