I think the pickups were the key to the great sounds out of those basses. If you have those? You'll be fine with any current, relatively colorless preamp on the market. The series you have was my favorite of ALL the TRB series 6ers I played. Those pickups were the key, and were used across Yamaha's 6 string lines for a long time. Aria's Steve Bailey signature 6 string basses used the same pickups, and at the time, both Seymour Duncan Bassline pickup series and Bartolini made pickups in similar shell shape. Duncan's preamps at the time were designed almost exactly like the Yamaha preamp, with a slap contour on a pull switch that acted very much like one of the 3 positions on the Yamaha preamp design. Both had identical treble boost hiss, and I think that the Yamaha and Duncan preamps were related, if not identical, circuits. You could probably find an older Basslines pre for cheap somewhere, but I think by today's standards, you'd be disappointed.
Bartolini make GREAT preamps, with plenty of options for control layout and function. I might go with a Bart pre myself, but today most folk fall on one side of a Love/Hate fence with Bartolini. I love em, many guys do not.
I am not a fan of the John East preamps suggested above. I've used them in several basses, and although they have many, many tonal options, that is an issue for me. Just too much going on, and too long to find a tone I desire, and extremely hard to adjust the same tone nightly. Consistency was difficult to find.
If I had to replace a TRB6 pre today, I would research the EQ points of the original, and look for a preamp with similar center frequencies. There are a TON of great options out there. Aguilar might be a choice for me. They have a 3 band preamp with selectable mid frequencies on a switch, that could probably feel similar to the original, but with longer lasting batteries and headroom enuff to handle the output of the Yamaha pickups, which are quite strong. Audere preamps are probably the least colored of any preamp I've tried, preserving the inherent tone of any bass I've installed them in, so if you love the Yamaha's tone, the Audere will preserve it and add power to any pickup configuration. I love Audere's approach and features. I've also installed a Noll preamp in a Warwick 6 I owned. That was a great preamp, but getting ahold of one may be tough.
Back when your bass was made, on board bass electronics were in an infant stage. Noisy, short battery life, bird's nests of wires or proprietary circuit boards made options very limited compared to today. Today, I don't think there's a bad replacement preamp made. Even less expensive options out there sound great, are easily installed and readily available. If you study just a bit, and decide what functions you want and what EQ points serve the tone you're after best, I don't think you can go wrong today in anything you choose. Having original pickups is key. You've got those, so try a few and see what you like.
