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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I had Strata III and replace with S3/SHO. Faster and more tuneful. There is the new T5x which may be as much as you need.
I have a REL 212/se in my main system. It was the easiest one to seamlessly integrate, and definitely the smoothest and most tuneful sub I've owned...for music (I've had pairs of SVS and HSU sealed subs in the past).  If the 212 is indicative of the entire line, REL is still a music targeted product, and not one I would recommend for home theater use, unless you're shopping their newer dedicated ht line.
No experience on mismatching of subs... but you will like two... two do more than just add bass, they will extended the soundstage.
@ghdprentice  My goal is to arrive at matched subs. It's a question of getting another (older) REL 328 or selling that and getting two newer REL models.

@noromance Thanks. Faster is better in my low ceilinged space.

@yakbob My purpose is music listening only. That's why I have the REL 328.