As I said about 1/3 of the way down from the top, the goal (at least in part) is to prevent sound waves from bouncing around off the bare wood walls. I'd suggest curving your pillow or whatever you're using around the speaker to block off the back, side, and bottom panels so reflections are minimized or eliminated. Just keep it away from the valve and chassis so air can circulate.
As far as what to use, try something that's already in your house for starters, a pillow, fibrefill from inside a pillow or duvet or out of some speaker you're not using, foam rubber, carpet pieces; and then see if you like the difference it makes, try more than 1 thing to see if that makes a further difference. Then you can consider buying something made for the job, or not. My VT30 still has a pillow in it and I'm very happy.

I just replied to you at http://www.valvetronix.net/forums/unhappy-with-your-amp-s-sound-read-this-t3774-30.html

Sorry about the double post: l

Yesterday I tried the open back mod at band rehersal. 100% open for now. I´m planning to do a 1/3 open back, Im working on it.

I percieved a little more volume, more room filling, a little less harsh " beam" and a little less bass.

I used a Strat with the Tweed 4x10... max gain, volume 12 o´clock, treble on 2 o´clock, mids on 10 o´clock, bass on 9 and master volume at 11 o ´clock, a little reverb.

Sounded good, not TOO much difference, but it works. I´ll cut a new back piece, 1/3 open, and I´ll try again.

Update: Last night I flipped out and went backwards: I stuffed the back of the amp.

I used the same stuff that Hydro used on the link that Cosmicrepairdude posted.

Now I can tell a difference. Same settings as before, tighter low end, crispier highs, more "kick" as we call it here. I also stood on the side of the amp, about 5 feet, as I did before, and yes, I notice a sound change but not as pronounced as before.

I´ll leave it that way and try it at rehearsal next week.

Hello all!
I tried this and yes amp sound better.
Real thing is to get it off the flor, to tune your guitar at high frequencies and to isolate cabinet.

When i did all this stuff, sound is much brither now, bass and mids are reducated and distorsion is better.
Thank you

Can someone by the way tell me whats the sound on uk90, cali etal, and boutique. On my and and guitar they are so dark. They ahve great distorsion but i just use them because they are so muddy and dark. If anyone has solution to this problem please say.
THANK YOU

Hi Guys whats all this fuss about I have a AD 30 valvetronix, I have took the back off and added a treble boost ...cranked it up and let it rip.....wow a completely different Amp .....no pillows ...no gaffa tape...just pure bliss.....

I also have this posted in Improved Tone From Your VT20+ but i also think it should be posted here as well.
I finely got around to putting some insulation in my VT20+,i found what looks like 3/4" or less appliance or speaker type fiberglass insulation that i cut into 5.5"x 11.5" for the sides of the amp,5.5"x 14.5" for the bottom and 7.5"x 13.5" for the backboard,i then cut some duct tape into six 2" pieces for each section of insulation and rolled the tape end to end with the sticky side out and applied it to the insulation to hold it in place.

This made my VT20+ sound much better then it did with only the beam blocker i had in it before i added the insulation,my tone is rounder and less harsh sounding now without the high end fizz that i got before,the high end is much smoother without affecting the midrange frequencies that much and i would also say that the midrange is a little smoother and better sounding then it did before.

When i first tried out my amp after putting in the insulation i still had some of my favorite user presets saved and i selected this overdriven blues type preset i used for my strat type guitars with the neck position pickup but i tried my modified schecter omen 6 with a GFS dream 90 P90 pickup in the neck position and i could not stop playing my guitar for over a half hour,it just sounded fantastic! I do not remember my vox sounding this good before putting in the insulation,my high gain metal tones also sounded better but still not as good as my vypyr high gain metal tones,maybe after i tweak my VT20+ some more i can get it sounding better for the high gain metal tones?

Yes,i would say this modification was a improvement and a success and would recommend everyone put in the beam blocker and insulation to improve the sound of there VT20+ and even the VT40+ if you think your amp is too fizzy and harsh sounding,it cost me next to nothing to do this and would recommend this modification first before trying a speaker change in your vox amp.