G'day mate, greetings from down unda!
Welcome to ML owners.
Re. To the bass drivers in the cabinets, yes they certainly can be improved by better bracing. There's no doubt about that. However, in this speaker series, there's no bracing, rather the sides of the box are glued. There are a few nuts and bolts or screws to put it but those are mainly for securing the back plate and securing the Stat panel. Hence, these are offered at a certain price point...
Even the Summit-X, Montis, Ethos, and Theos series, which was referred to as the Reserved line, if memory serves correct, didn't use heavy bracing either. It was only later introduced in the Masterpiece series.
If you consider the Masterpiece series (Ren15a, 13a, 11a & Classic 9) and audition any of these, at once you'll notice the difference straight away! The transparency is taken up a notch further and the power and dynamics are several gears shifted upwards. Really powerful hybrid designs yet delicate like a butterfly's wings... and they can play louder, reproducing dynamic peaks that seem limitless. And, these systems cost way more than any previous hybrid series ML ever made. Their top of the line being the Neoliths that retail in Aus for 160grand!
So, unless you're going to do a major overall, in which case it's going to cost you a fair bit to redesign these cabinets, might as well save up and go for any of the Masterpiece series when finances permit...
Re. To bass drivers / woofers not sounding so great as compared to before, yes this can happen in the older hybrids. This is mainly due to different types of materials used back then vs now. Back then, it was mostly paper cones, combined with stiff fibres to give greater stiffness but over time these wear out.
With new materials that ML uses in all their bass drivers are pure aluminium, and high grade carbon alloys. These can outlast any form of fibres or treated pulp, and they are highly rigid yet extremely light. So the speed on which they move is completely at a new level compared to any of the vintage hybrids that ML produced.
Just my 50cts worth...
If you can refurb the bass drivers at least, that may help but bracing the cabinets... you would have to take the whole thing apart and rebolt the joints, plus brace horizontal locations... this will be tedious. Moving forwards, I would highly recommend any of their new Masterpiece hybrids.
All the best, let us know how it goes.
Cheers, RJ