Apparently my MIG-100 was all original. The plastic standby switch finally broke a few weeks ago, and I took it to Matt Weed of Atomium Amplification to replace both (the power switch was three-way for some reason). Here is what Matt has to say about them in his post.

Sovtek Mig-100, all original except for the new power/standby toggle switches I installed (the plastic originals are awful and break all the time). Not exactly a Marshall 2203 but very close. The distinctive sound of these amps mostly comes from the circuit tweaks Sovtek made to get more reliable operation from Russian military tetrodes – 2.2k screen resistors, for example, to limit screen dissipation. The B+ is 520V!

I love this amp, it’s loud, sounds great, and takes pedals well. Easily my favorite amp.