Older vs. newer REL subs

"I'm considering the T7x and T9x lines."

"But as often happens on this site no one actually answered the question that was asked in the op."

Unless you can get that 8 year old technology for a killer deal(at least 1/2 original retail), a pair current Tx is the way to go. The current Tx series is probably closer to the S series now than your current sub, which is still a capable sub. It also has a nice modern look.

My R.E.L B3 is stone age compared to even the "value" T series of REL. 2  completely different companies. Your sub is from the Sumiko years, and since then to present is its own company with even nicer offerings.

Why not a pair of Salk Sound Rythmik 12’s in the same finish as your Salks...

The PEQ will come in handy now that you are taking measurements.  The cabinets look wicked rigid too.

As far as Rel, I’ve had a T9i, an S3 SHO, and a T5i.  The T9i is pretty sweet and I imagine the new version is even better.  I’d imagine that you’d be pretty stoked with a pair of T9x’s.  

I do like the idea of newer subs. A Salk sub would be nice but 75lbs each? Probably harder to resell, no?

Yes, probably slightly (maybe moderately?) harder to resell the Salk but who knows.  Hopefully you wouldn’t want to sell them!  
And yes, 75lbs sucks to move around so theres that too.  Also, how long would you have to wait? And then there's size.  Quite a bit bigger than the T9x too.

I don’t bro, your call.  I do think a pair of T9x’s would be pretty sweet though.

The good news is that my 3 REL subs from basically the same era, two Q150e 10" masterpieces (had one for years, just bought a second one) and one Q108MK2 8" all work beautifully and were bought used el cheapo. The bad news is that REL won’t service them. If one dies I would try to find somebody to fix it, but they’ve essentially paid for themselves relative to what new ones cost. In my home studio pile I have a 92lb beast Mackie HRS 120 (not a portable P.A. Mackie, this thing is far more adjustable) with a 12" woofer and 12" passive radiator and 400 or so A/B watts that goes down to 19hz...I found it at a used instrument shop for 200 bucks and I’m pretty sure they didn’t know how it worked...a lot of woof per dollar...I don’t think subs necessarily have to match as I’ve used the 150 and 108 together in my hifi rig for years and you simply have to have them dialed in properly.