My advice is to not worry about tubes amps until after you’ve had plenty of time with the 6XX on the Element II. I own Crack and Tuba tube amps and I still prefer the 6XX on solid state. Though that’s mainly because my favorite tubes don’t tune the 6XX to my preferences.
Depending on the amp, you can get some subjective improvement using a tube preamp. Here’s a thread on the subject. They’re cheap, so it doesn’t hurt to try them out. But I wouldn’t bother unless you really think it might tweak the sound in the right direction after listening to the 6XX and element II