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 have both a G2 and a Stadium lll and each has attributes that are unique to each. The G2 is fast enough to keep up with the 6 servo controlled woofers in my Genesis G500 speakers but I had trouble blending it with other speakers. The Stadium lll on the other hand has extension that provides a different acoustic signature and its speed blends better with most speakers. Think of the G series as a Porsche and the ST series as a Cadillac. The Stadium lll provides solidity and air and the G2 adds speed and definition.

Hope that helps
tablejockey - The BK sub is pretty good and won't give you a hernia like the REL. The REL's are faster and clearer. Depends how much cash you want to drop and whether your back can handle it.
nmmusicman - Great analogy!
My rule of thumb is ten years makes for a significant improvement in sound from a line or company or a technology (high-end stuff).
I read through this thread because I have a somewhat similar question .  But as often happens on this site no one actually answered the question that was asked in the op. 
I have rel r328 and S5. The S5 is quite a bit better, as you’d expect being more expensive and newer.  Different systems. Ive never taken the time to A/B just my impression being owner of both