Prof - I’ve only heard the 3.7 and 2.7 once, when auditioning the final prototype 2.7 at the Thiel factory in 2012. We all heard what you are hearing.
There’s probably more to it than the baffle. A few years and $six figures were spent optimizing the 2.7 within its budget parameters. The aluminum baffle is a contributing element, but any upgrade would have to be from outside, since the interior has multiple shelf braces in the way. A person might route a pocket into the MDF baffle front to seat a custom aluminum plate. Serious undertaking that I doubt Rob would take on - but ask him. You would get significant improvement using counter-top laminate (Formica, etc.) rather than aluminum.
An effective and feasible upgrade addresses the 2.7 midrange xo feed which goes through a 400uF electrolytic cap as well as a series 20 gauge feed coil, without any shunt to ground. Jim never used electrolytics in series feed stations (after the 02 in 1976). That "caught in the box" effect is something that big E caps do. I have developed some substitutes from Clarity Cap 100uF x 160 volt CSAs. I doubt you have room in the enclosure for them plus they’re fairly expensive. Note that Jim’s solution for the 3.7 is a cap bridge with multiple 75uF PPs. The feed coil in that station in the 3.7 is 18 gauge for about double the current / resistance performance.
If I were in your shoes I’d consider the following:
Baffle treatment: remove the drivers and the threaded inserts (if any).Mount 1/2" (or larger) birch dowels into holes drilled into the edge of the driver openings, at least an inch and preferably farther behind the front surface. Wood screws into that side grain of the dowels will couple the drivers more tightly to a greater cross section of the baffle. Use viscous gasket goop behind the drivers (Permatex type 2 non hardening) to damp the natural interface resonance. That mounting on a formica face gets you pretty far up the performance ladder.
Crossover: replace that 400uF E-cap with 4 @ 100uF in parallel. Consider replacing the bypass caps with ClarityCap CSAs. While you’re in there I’d swap in some Mills MRA-12 resistors in that series midrange feed. Same for the series feed in the tweeter.
That’s some low hanging fruit and something that a good bench tech could handle if it’s more than you want to tackle. Rob may have suitable parts or advice, or you may contact me for specs and sources.
The 2.7 is a very nice speaker and in some ways easier to take on a broader range of material and amplification than the 3.7. But immediacy and detail are relatively compromised.
