Hey Micky, Rule of thumb is to get a UPS such that the power you need is ~60% of the UPS's spec. So if you need 600VA to comfortably run you rig, get a 1KVA (1000VA) which is what I did. My rig is similar (MP2, Quadverb, Midiverb4, mixer, amp (either carvin TS100, or carvin DCM200L or B200s). I've since added a MB1 as well but I've scoped it for plenty of wriggle room to allow additions.
You can get the power consumption from the units spec sheet (manuals). The old depot gear pages are still available here (http://web.archive.org/web/20040422213430/http://adadepot.com) and you can navigate to the various units.
MP2 uses 35W, MB1 uses 12W (according to their spec sheets)
Conversion site (http://www.powerstream.com/VA-Watts.htm)
The conversion of W to KVA is governed by the equation KVA=W/(1000*PF) If you go with power factor of 0.6 then
So for MP2 KVA=35/(1000*0.6) > KVA= 35/600 KVA = 0.0583 or 58 VA
So you could (Very) conservatively assume each small 1RU effect/preamp style unit is no more than say 60VA. Above I have 5 such units (60*5 = 300VA (with plenty of wriggle room)). My TS100 says it's 300VA, so I need 600VA all together.
You'd most likely get away with a 550VA UPS as your biggest draw is the power amp and my number above have lots of wriggle room. But I'd go for bigger (750VA or better 1KVA) as I have done