REL subwoofer questions, Storm III and/or Stadium II

Hello all - I have a couple questions re REL subs - 

First, is there a downside to hooking RCA connectors up to the bare wire ends on the OEM cable and use a preamp output rather than the amp?  This would be done only for convenience.

Second, how bad of an idea is it to mismatch these subs, meaning having one Storm III and one Stadium II?  Again, this is a convenience/patience issue for me - I bought a used Storm III about two years ago, fully expecting to eventually pick up a matching second one, which has not happened.  In the meantime, I've seen several Stadium IIs on the market, and I am impatient to hear the impact of two subs.  Should I continue to be patient or is the pairing acceptable?  If I did this then I'd be itching to eventually get a matched pair and either have three subs or get rid of the one that doesn't match.

Thirdly, how much of a difference do upgrade power cords make with these subs?  Is it worth another $200-$300 each for better cables?  

Thanks ahead of time for sharing your insight and experiences!

Rest of system: PC -> Bel Canto 3.7VB DAC -> Sonic Frontiers LSF1 Preamp -> Krell KAV 350il (in HT mode) or BAT VK-55 -> Thiel 2.3s.  Room is 9' x 20' x 9'.  Music is mostly Americana, alt/indy rock, jazz, chamber pop.