First of all, the Shiva 20th is a different beast. It has a lot more gain than the stock Shiva. It's tone is wider, with more low end, more clarity, and more overall harmonic detail. Without pedals, the Shiva 20th can do high-gain but the Shiva can't. With pedals, the Shiva can do anything.
The Shiva isn't like a JCM 800. It has some more gain and can give up good tones at low volumes. It has a clean channel that can be pushed beautifully with some volume. The Shiva 20th has a clean channel with a boost, so you can get that plexi-like breakup without so much volume.
The Shiva has a distinct midrange with prominent lower mids and a kind of smoky high end. The mid shift of the Shiva works extremely well. You can get closer to a JCM800 when it's engaged because it kind of scoops some of the low mids and lets some of the high mids stand out. It also reduces the volume slightly, so even better a low volumes. Both Shivas have this feature.
I wouldn't recommend the Shiva 20th with EL34's. In my opinion, it's KT88's or nothing. The Shiva can be bought with 6L6 or EL34, but not interchangeable. I think the 6L6 Shiva is slightly better overall because of the way the 6L6 power section shapes the midrange. It's slightly less dominant in the lower mids than wiith EL34s. However, there is a certain appeal EL34s have when you're pushing the volume.
Reverb or without reverb doesn't matter to me. I personally love Bogner's reverbs. I suggest disregarding all the B.S. about the silver back panel and all that jazz. It's all bullshit in my opinion. Unless you're one of those real specific guys who knows EXACTLY what you're looking for in the circuits or something.
I wouldn't necessarily call the Shiva 20th more modern. It's not as hi-fi as say a Diezel. In fact, it's pretty classic sounding in all its plexi type tones. Either version has an incredible clean channel, but the 6L6 version of the stock Shiva might be the best cleans. It doesn't take the Shiva much overdrive to achieve high gain. It's a lot like using pedals on a Friedman Smallbox. It doesn't sound like a Smallbox. It's thicker and warmer in the high end than a Smallbox.
If 80's tones are kind of your thing, the Shiva will do if you don't have to play at whisper volume. If you need lower volume or if you need higher gain, you'll need a boost or overdrive. There is a lot of magic in the Shiva. If you want a one-stop-shop, the Shiva 20th will do any kind of music. It's as versatile as an Ecstasy.
). Wish I could help with the rest of your questions, sorry.
