Second Pre recommendation 500$ budget

First, I haven't used/heard the Metric Halo preamps, so can't really compare, but I'd also give a hard look at the AEA TRP, or other well-engineered/regarded clean preamp. I recently got a tube mic, and the AEA preamp is neutral enough to really hear that mic whereas the BAE 1073/API and tube pres I've put it through veil its detail a little more than I personally like for my own recording. Your GAP ribbon and top shelf dynamics will sound very different through a really nice clean preamp like the TRP, and ESPECIALLY on things like percussion and the other sources you mentioned where clarity can be desirable.

I'm sure the MH preamps are good, too, but if you have patience/interest, I'd suggest at least getting the nicest outboard clean pre you can afford (with a no questions return policy) and comparing. I love my BAE, but since my main condenser is already colored and I tend towards ribbons on drums/guitar amps/vocals sometimes, I'd personally be lost without the TRP. Lately, my BAE 1073 has been nothing more than a bass di (and a very good one). I thought my Apollo's mic inputs would be good general purpose "clean utility" preamps, and they're not bad, but there's not denying the outboard "clean utility" pres I've used sound better. 

If you do decide you want color, I'd definitely cast a vote for any of the Dakings. I have only CAPI and other vintage-y API clones lately, but I remember the Daking Mic Pre One to be quite a bit more neutral than those. I did have a 512 awhile back and it was definitely more neutral than the vintage/312 clones that I had at the same time, too. Again, I really liked the Daking and kind of wish I'd kept it. I remember it as a quintessential "neutral but not clinical/boring" sound...it sounded 'Classy' which I know sounds a little silly, but I really remember it that way. Someday, I'll try out some John Hardy's, which a lot of engineers I've known over the years swear by for "neutral but not clinical/boring."

The only other color pre I remember really loving that sells for a lot less than it should used IMO these days is the Great River MP1-NV - Great River's take on the 1073, but "dehazed"...it was built specifically to channel the girth of the 1073, but without quite as much build up in the mids. As I understood it, when that pre came out, that sort of mid build up was a common criticism that engineers made when tracking projects entirely through 1073s.