REL subwoofer questions, Storm III and/or Stadium II

m-db = thanks a bunch for your detailed response, you've greatly increased my modest knowledge of these 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.

The REL Neutrik Speakon HI INPUT [9] is designed to be connected to an amplifiers high level / speaker outputs.
The Storm III's RCA LO INPUTS [6&7] are used to connect a preamplifiers right and left RCA outputs.
A 12 dB attenuator is available from REL to balance the two inputs.
Alternatively, a high quality Y-adaptor may be employed to connect at either input as system gain characteristics demand. 
The XLR BAL LO INPUT [10] is for a preamplifiers balanced XLR output.
When either XLR or RCA low level signal from a preamplifier enters any of the Storm III's LO INPUTs an amplifiers input/output stages, and the REL HI INPUT circuit are bypassed and REL's proprietary Neutrik Speakon terminated cable is not used.
Monoprice or Blue Jeans Cable offer economical custom length terminated cables and Y connectors. 

Second, how bad of an idea is it to mismatch these subs, meaning having one Storm III and one Stadium II? 

It's not at all a bad idea.
REL "Sub-Bass Systems" have gone from your British hand made Storm to being  sourced off shore essentially using the same decades old circuit. REL "Systems" are typically down -6dB before their moderate actual in room response.  
Consider companioning your Storm with a subwoofer equipped with low level outputs to daisy chain from your preamps outputs into an EQ and/or the input/output subwoofer, on to your Storm III's RCA LO INPUTS [6&7] instead of any other ST series. 

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

tablejockey, I agree. 
In my experience with a Studio III I highly recommend considering some form of equalization such as MiniDSP Dirac, REW, and a calibrated microphone for the equalization and control of most any sub, speaker, and room combination. I found a hugely more noticeable improvement compared to the high level connection and the most exotic cabling available. Good luck M.

I think nmmusicman is on to something with that suggestion. I have a B1 and a Storm III, and occasionally use only the B1 high-level (with speakers not requiring it). Storm III ended up in TV room with Elacs.

I don't have experience with newer R.E.L. subs but recently bought two SVS SB-2000s for a theater room, and one new gloss SB-3000. I thought I might replace the older ported B1 as that SB-3000 is quite a sub (esp for $). But as of now the B1 is still in the system, if rarely ever used.

Slightly off topic, but how do the old Stadiums/Storm/Strata’s compare to the much newer and higher power REL subs for music? Those are 10+ years old. 

Just another perspective.

Don't waste money on PC/aftermarket cable. I've used REL for 10 years. The stock PC and speakon cable will do the job. Using the Britannia series, which is the last of the actual British REL.

Cable being a YMMV thing, the aftermarket PC plugged into it doesn't do anything different than a supplied PC. Same goes for the suppled made in China  speakon cable.

Thanks!

both are great... stadium more output but i have 2 storms and in tandem they are tremendously musical and load my 19x17 room with ease

rel's for two channel audio always best to use high level connections and the speakon connector ran from your power amp

You can use your subs to great advantage by using them in a mono configuration with the Stadium ll up front and the Storm lll in the rear and tune them with the controls and placement to mitigate the modes in your room. I use this very setup in my vinyl room with a Stadium lll in the front and a Strata lll in the rear hooked up in mono with great results.

Yes power cords make a difference with these subs as do the high level cables and any money spent on either or both returns dividends. As for the using line level into the high level inputs, I don’t think there would be anywhere near enough gain. Curious why you don’t use the speaker terminals??

Cheers