If it were me, I'd skip/sell the Mixpad and go with the UA5. A few reasons I personally wouldn't go with Mixpad > ADC:
[1] I'm mostly a fan of less gear = fewer hassles with minimal, if any, compromise on sound (depending on gear combos, of course)
[2] I don't think the Mixpad sounds much, if any, better than the UA5 pre
[3] I'm not a big fan of the standalone ADCs out there, namely:
- AD500 (sound great, but old and unreliable)
- ADC-20 (great detail but bright/harsh and digital sounding to my ears)
- SBM-1 (stock not better enough than the D100/M1 ADC to warrant the cash, and for the same price you get the much more flexible UA5)
- modSBM-1 (my favorite of the bunch, but a bit pricey - I'd upgrade mics before picking one up)
Reasons why I personally would go with the UA5:
[1] sounds good with the Oktavas if you're considering those mics - should be plenty of recordings around to give 'em a listen
[2] I prefer the UA5s ADC to the readily available inexpensive ADCs (see above)
[3] upgradable: digi and preamp mods from Oade if purchased direct - or DIY if you're especially handy at such things (FWIW, members of this board were split about 50/50 during blind testing between warm-mod UA5 and V3)
[4] Much broader feature set to allow you to grow into the hobby:
- 16- or 24-bit
- selection of sample rates
- variable gain
- ability to run AUD + SBD or 3-mic mix
- both optical and coax output providing flexibility in recording devices (e.g. DAT, JB3)
FWIW, the UA5 gets "fluffed" a lot because it's probably the best price/performance value in pre/ADCs for our application right now.