> Ford did a trick in the 80's where the rear ends of their mustangs would vibrate and you could hit a speed that would cause an oscillation that felt like the thing was coming apart..
Not quite.
Sit in the back of a Chevy Nova for a day-trip to DC. You don't notice, but there is a booming noise from the trunk which gives you a headache.
Somehow Ford noticed this on the essentially similar Falcon. Cost, weight, was totally out of the question. Instead they lightened the rear "frame" rails to shift the resonance. I suspect the Nova had "frame"/floor resonance amplified by trunk air-volume resonance; the Falcon/Mustang metal thunked a higher pitch than trunk volume boom.
The Cougar went further, with 200 pounds of clay/tar mats under the back seat.
The flip-side of lightening the "rails" is that after a couple decades of rust, you can put your finger through the rear "frame" rails.
The Accord does have a rubber-wrapped lump on the front frame, damps a something or other vibration. And one of the engine mounts is hollow with an electric valve: traps the air, or not, under ECU control. I have not figured out if this is supposed to tame the idle-shake, or let the engine float free at high RPM and light load, or what.
> I just don't see how putting a lightweight damper on the glass
Touch a finger to a working tube. Very light touch can make a real difference. I see what you are saying, you want to GRAB that tube with an 800 pound gorilla. But much lesser dampers can have real effect.
Remember are not getting much vibration "through" the glass. The whole bottle is poorly supported on 9 sorta-tight socket holes, and shakes easily. The "shields" often used on older gear can be electric field shields, but they often suppress bottle vibration too. Rather, some do and some don't.
As Charlie says: "damper rings" are three for a dollar at McM-Carr or the NAPA parts store. Most NAPA have a tray of O-rings, rated for engine heat. Any of these will be cool on 12AX7; a full-heat 6550 may toast the general-purpose Neoprene but there are Silicones which stand up to more than a tube seal can take. You wanna know OD and thickness, which will mean some reverse calculation from tube diameter. If they like your look, they will probably let you play with your tube to find a fit you like.