Stereo RELs, two different sizes?


I’m running a pair of REL subs but they’re not the same model. One is the S/510 the other is the T/9i. I have them positioned so they’re opposite to each other meaning the larger S/510 is in the right front corner. The other smaller T/9i is just to the left of the listening position. I have them pretty well dialed in regarding the level and crossovers for each. Sounds great! My question is what I might expect to hear if I was to replace the T/9i with another S/510 so the subs are the same size/model? It’s sounds amazing as is but had always intended on getting another 510 because that makes more sense to my brain but it sounds great as is so why bother? Probably just answered my own question but I’ll hit post anyways. Thanks! 
paulgardner
An alternative is to use all 3 subs. There is a vocal contingent advocating the subwoofer swarm idea. I haven’t tried it but I am considering going from 2 to a 4 REL stack. The challenge is the doubling of crossovers and gain to optimize. It took me a month or two to dial in the first pair of subwoofers. I worry that dialing in 4 will be a PITA. 
@fuzztone Ha yeah. I only say stereo referring to more than one sub. I believe Rel calls two subs “stereo subs”. Not claiming I hear stereo bass. 
Unless you are overdriving the T/9i, and I highly doubt you are, it is highly unlikely you will hear any difference if you replaced it with a second S/510.
I've one S3/SHO—the earlier 510. I've shied away from the expense of adding another. Your experience brings the possibility of adding a T7/9 instead. Thanks!
I've been using 2 dissimilar RELs for years to great effect...older models from the "Q" line but they work like new. A 10' front firing Q150e and an 8" down firing Q108MKII...neither has a passive radiator and they work together beautifully.