I believe the Studio Series and the Studio .22 are basically the same circuit. The Studio .22+ differs from the non-plus version in that it has a separate lead master volume control. The schematics for the two versions have very few differences other than that.
I was actually in the same boat as you...I have a Studio Pre with a Simul 395 that I love but I needed a smaller option. I looked around locally for a Studio .22+ and wound up finding a Studio .22 without the graphic EQ. I just won a Studio .22+ with the GEQ on eBay, and I'm planning to keep whichever one I like better. I'm expecting to prefer the .22+, based on the added flexibility of the lead master and the GEQ (without the GEQ it is a VERY midrangey amp), but we'll see. The .22 definitely sounds great in its own right. However, I don't really find the character of the amp to be very close to the Studio Preamp. The lack of a lead drive control makes it pretty much impossible to get a pristine clean and a smooth crunchy lead with the same channel settings. The EL84s in the power amp give the overall tone a very different character than my rack setup has. It's a cool character in its own right, but different. If you set the amp to optimize the rhythm mode for clean, you get some real compressed Vox-y tones that always have a little bit of dirt in them (with humbuckers at least), but sound real nice. You can get far more gain out of the lead mode than I could imagine necessary. But with the EL84s in the power amp, it doesn't sound like a Mark so much.
I will definitely be keeping either the .22 or the .22+, but it's not a simple drop-in replacement for a Studio Preamp.
