Great looking amp and I'm glad to learn that it sounds great as well. How'd you polish the top plate? And how'd you polish it so fast? Mine's primed and painted.
I try not to waste any time and start very aggressively.
Of course, wear ear, eye and breathing protection.
I started by mowing off the wire brush scratches with some "Sandblaster" 120 grit (I think it's a 3M product) on a palm sander. That stuff is hell on wheels and if you used it on wood, it could gouge in an instant but for removing some mil's on metal, it's great.
It does leave major swirl marks so I then went down to 150 grit aluminum oxide then 320 grit. All that took me about 1.5 hours. The last sandpaper I used was some more 320 grit that I rubbed against each other and that makes it closer to 1000 grit.
With each grit, stop after a few minutes, wipe off the surface with a rag and look to see what "new" swirl marks you're making and what you're getting rid of. It doesn't pay to go any further than the grit will allow.
After the sandpaper, I cut up some rags into squares and put them on the sander. I smear "Mother's Aluminum and Mag Polish" (at your auto parts store) and polish it till most of the sanding swirls are gone. This means every 10 minutes or so, you need to clean off the polish and take a soft clean rag and polish. You'll be able to see when most of the sandpaper swirls are gone and just the very fine polishing swirls are being created now.
When you get to the point that you aren't making any progress, take a foam car wax applicator (I like Meguires), apply a light coating of the Mag polish and using a very light touch in circular motions, get rid of as much of the swirling as you can.
Polish it with clean, soft cloths and wash with 409/fantastic type cleaners and water. Examine closely at every angle.
If you weren't super careful, you may find marks from earlier sandings. In this case, it's better to just give up and go back to that point in the polishing rather than wear your arm off trying to get it with too light a finishing stage.
I then put clear gloss lacquer to protect the polishing (it'll scratch by just looking at it if you don't). If it's a flat surface like this plate, I flood it on in one coat (just short of dripping off the edges). Of course, keeping everything totally clean and dust free is key to the final finish you'll get.
Wear a respirator if you value your brain cells too.
I guess I spent about 3.5 hours doing this and painting the transformer bell.
This process won't yield a totally mirror finish but this isn't silver we're working with. You can totally remove any swirls by using the compounds that are used to polish the mirrors in reflecting telescope kits (Edmund Scientific) but I don't think it's worth the extra time.