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.
128x128hilde45
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 
@kren0006 I think the R328 was a more expensive model at the time and so it's at the same level, perhaps, as the S5, now? If so, that's at least a rough analogy.

I'm not in the market to buy two $2500 subs to replace a single older $2k sub; I would entertain two newer $1500 subs, however.