The backordered Groove Tubes 928-PRs finally came in today; compared them to the THD YellowJackets in the Blues Deluxe; both converters have GT-EL84-S in them; Sovtek 12AX&WBs in preamp slots, vintage Fender 7025 in phase inverter slot.
1.) See pic below; GT-928-PR converters are almost 3/8" shorter than the YellowJackets, might fit in some heads with limited space the YellowJackets won't.
2.) The Groove Tubes sound a little different than the YellowJackets; YellowJackets sound a little darker, GTs a little brighter. I was going to resell whichever pair I didn't like as much, but like both and the difference in tone is enough to keep both.
3.) As a function of how these work in a 6L6 based amp, you get "max volume" on the YellowJackets at about 5 on the Blues Deluxe volume dial, about 6 with the GT-928-PRs. You can turn the volume all the way up, but it doesn't get appreciably louder past that point - CB can probably explain why if you care...
Hard to recommend one over the other; one's brighter, one's darker; max volume is the same and through a Celestion Vintage 30 slightly louder than a stock Fender Blues Jr.
Also note that with the stock tubes in the Blues Deluxe I've never the "Drive" channel other than playing around with it the first couple days I had the amp, as I REALLY didn't like the distorted sound; I've always just used the "Normal" channel, and know a lot of people feel the same way about the Blues Deluxe/Blues Deville/Hot Rod Deluxe/Deville. With either of the EL84 converters subbed for the 6L6, it kicks ass, dial the normal channel down a bit for a cleaner sound and kick into "Drive" at near max or max for lead Crunch City. If you own one of this series of Fender tube amps you need a set of these converters, great for all the smaller gigs/venues. The sound level meter we use at work (I'm a little dubious of it's accuracy at volumes over 100dB) shows 111.2 dB with the amp pegged with converters; by way of comparison the stock Blues Jr, reads 109.8 dimed out.