No, I am not modding the cabinets at all. Right now I have just damped the horns and woofer frames, and really improved the crossovers with far better parts. If I ever wanted to sell the speakers I have not devalued them in any way, but rather I have improved them. Cutting holes= end of value. I will say that putting ODAM caps in and putting the pathaudio (or other fine resistor I suspect) is a big difference.
No never heard a Tannoy of any sort. Right now I need to put the spatial x5 back in at some point and compare. Two completely different solutions, both with their strengths. The Cornwalls are fun and they do pretty much everything right, but they do bottom out at the very, very low end. Really it isn't an issue. I hate subs as they are tough to integrate and there is always the temptation to fool with them, but a pair of subs to just get you 20-40 Hz reinforcement would do it. You hardly need them with the cornwalls, just on a few tracks that have a lot of energy in the really low end. A piano lowest note is 28 Hz as I recall. The X5 integrated subs were perfect. Once the woofer surrounds loosened up I dialed them down a wee bit and never touched them. Clayton has it worked out.
Still, as you have discovered, the cornwall IV is just tons of fun and you really have to ask yourself if you need a better speaker. Just enjoy the music and don't worry about it. Put a good tube system in front of them and be happy. If you do bother to improve the crossover parts you will hear it, but they are quite nice stock.