Older vs. newer REL subs


Looking for any reports about older vs. newer REL subs.

I have a REL 328 (ca. 2013) which I bought used. It's a good sub.

https://www.hifiplus.com/articles/rel-r-328-subwoofer/

I would like to upgrade to stereo subs and I've found (finally) another REL 328 which would match mine. But, before I drop coin on that, I'm wondering if anyone has had experience comparing their older REL with a newer one. What's changed? What's better, worse, or the same? I'm considering the T7x and T9x lines.

Hoping for apples to apples comparisons, vis a vis driver size, type of sub (e.g. level of product line, sealed vs. ported, music vs. HT etc.) but whatever old vs. newer comparison you have much appreciated. 

Thanks.
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@stereoisomer Thanks. I did a lot of homework on the matching sub question and this is why I still have one sub. Because it had to match. Now that I've found a match, it's the right time to ask, "newer or older" matching subs. It would cost money, but now is the time to decide.
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Keeping the model, year, version the same with dual subs is smart. Buying a pair together closer in serial number helps ensure there is less chance of variation.
Personally I would buy the matching sub. That being said, the newer ones do perform a bit better.
Speaking of older REL subs...

Can anyone provide any insight to the BK electronics subs? They were the OEM builders of the orignal REL models?(as I understand, clarify if necessary)
https://www.bkelec.com/HiFi/Sub_Woofers/P12300SB-PR.html

They seem reasonably priced for what  they are. Not as high tech as the current REL, but seemingly competent. Looks like they still use the same driver from 15 years ago.