Got a Rockmaster and E530 here. Both great units.
The Peavey isn't quite as modern sounding and is rawer. Has tons (way too much!) of gain and a quite flexible EQ. It's the pre I use live all the time. I don't like the clean on it much at all, very bland but I don't play with clean much so I'm happy! It's really a great sounding pre. Recently used it for a more 70's styled prog/heavy blues type thing live ( Deep Purple/Toto) and it sounded awesome at that too.
The Engl I've got here is my brothers and it sounds great. Quite a modern sounding unit and has a smooth distortion. The clean on it is also nice enough. It's not as flexible as the peavey with its range of distortion tones but its sounds great at what it does.
Also got a GSP1011 and a Hafler T2 pre here. The Gsp is actually a great pre, especially for live use and since the firmware got updated to have a better EQ and more models. I reckon its better than a pod and I would consider it a poor mans Axe FX. Doesn't do the slightly distorted tones that well but excels at the high gain saturated tones and the cleans on it are awesome. Sounds good and you can upload your own impulses onto it. Also has a really good effects section and sims alot of OD units too for boosts- couple of tubescreamers, boss units, digitech ones etc... A very underrated piece of kit, I'm very surprised alot more people aren't using them.
Both the Engl and Hafler are out of action t the minute- only the clean works on the hafler. Been like that since I got it as a result of a crappy postage job on behalf of the guy who sold it to me. The clean is bland enough but I've not heard the Od channel, its doesn't have a very flexible EQ - no mid control! But they can be had for cheap. Actually sounds great for clean bass tones, which is why I've not bothered getting it fixed yet, I use it too much.
I love the idea of getting a lot of pres for guitars. I definately want to pick up a Mesa studio pre at some stage. Would round out the selection nicely, as I've got a JCM 800 and Laney GH5OL covering the slightly more old school tones. Definately look into getting a nice tube power amp. Got a Marshall 9100 EL34 loaded beast here. 100 watts a side and it slays. The mesa 50/50's also seem really good but I've yet to play through one. I do wish someone made a simple affordable mono 50watter tube poweramp as almost every unit is stereo which works out great for live( me and my brother share the poweramp and house the 2 pre all together in 1 handy 5U rack) but really is overkill for the studio.