FWIW, I just drove into the local independent muffler shop, and told him the old system was too loud for my tastes. He sold me two "Cherry Bomb" glasspacks in series, very similar to the factory system except round instead of oval. Cost was very minimal, and included installation. With an appointment, it only took him about 30 minutes to install. I'm very happy with the result, still sounds like an old sports car but no longer sets off car alarms in parking garages and so on.
After finding how easy (and basically free) having him install my exhaust systems is; I'll never do my own again.
Also FWIW, with the previous single glasspack, I discovered that a downturned tip makes it seem less loud. Apparently bouncing the sound waves off the pavement helps spread them out more, or something like that. Anyway, it was a quick, cheap fix and I ran that way for many years.
The stock system (as well as the ones I bought) uses fiberglass filling to absorb and muffle the sound waves. Over time, the filling breaks down and finds it's way through the perforations and out the tailpipe (aka "burns out") and you can wind up with a hollow chamber that does nothing to muffle the sound (and might even make it louder). This photo is from a Stag, but the stock TR3 unit is exactly the same construction.