I have to admit, this thread intrigued me. But it also has me puzzled to.
If you're starting off with "really excellent" speakers, then the cabinets themselves are going to be really good as well. The mid 80's to early 90's marked a watershed point in the speaker industry. Affordable computing became available, and with it even the smaller forms were able to start using CAD/CAM to model and predict speaker cabinet performance. MDF replaced chipboard as budget cabinet material and CnC wood milling brought low cost mass production techniques to cabinet construction. The days of best guess, gut feeling and dodgy boxes was dwindling fast. Celestion were even experimenting with high rigidity aerospace materials (Aerolam?) in their SL6 and SL600 speakers. So, other than perhaps re-veneering, where exactly is the benefit of fixing something that isn't broken?
That then leads on to the question of whether you'd follow the original design specs or change the cabinet volume. Doing the latter means you change the fundamental characteristics of a speaker's sound. I'm sure you're already aware that speaker design is more than just slapping some drivers and a crossover in a nice looking box. So by messing with the recipe for a proven design, isn't that more of the same fixing what ain't bust, or worse and going backwards?
The kinds of 90's and 00's speakers that could be improved with better cabinets are perhaps the ones that were never really that good in the first place. Cheaper floor-standers for example. However, unless this is a one-off project for yourself then what do you plan to do with such souped-up speakers after? Getting your money back just for materials alone would be a challenge when the used market is already reasonably well supplied with good quality and well reviewed speakers.
If the project is going to be worthwhile I think you have to hit the heyday of the British Hi-Fi scene and look at designs from the 70's and early 80's. This is when the business was still largely a cottage industry. Cabinets were made from either solid wood or chipboard with a vinyl wrap. Bass reflex was becoming popular which broadened the appeal of the standmount speakers but the industry was still experimenting with the technology. Good entry-level speakers were around £100-£150 then - equivalent to £300~£450 in today's terms.
Back in the early 80's £99 was the minimum spend for Hi-Fi performance (Mission 70 MKII) and the step up was the £150~£200 range. Popular choices were Heybrook HB1, JPW AP1, Acoustic Research AR-18(?), Tannoy Mercury M20. Some have withstood the ravages of time better than others. Heybrooks, AR’s and JPW’s all used foam ring suspension for the bass driver which perishes after about 20 years. Tannoy used a rubber suspension. If it’s not abused, then the drivers in a 30 year old pair of Tannoy Mercury or Venus speakers will still be going strong today.
Some other speakers from the 80's 90's which might be worth a look
Rodgers LS 55
TDL RTL2
Castle Trent / Pembroke / Chester / York
Epos Acoustics ES11
Richard Allan Pavane
Tannoy Mercury M20 / Stratford
KEF Concerto
Linn Index / Sara
Then there's a whole slew of Celestions from the 70's/80's