i suppose we can discuss the original question.
xtant - i've ran 1999-era xtant amps in several builds and owned them since 1999. awesome amps for flexibility but difficult to find mounting locations due to cover design and fragility. active capable. great sound and power supply design. 3300c have thermal power supply issues. 403a 603x 604x 1001dx are all great choices. avoid x603, x604, x1001 as they are MTX amps and it shows.
phoenix gold - i've owned several PG amps, but if i did an OS build i'd probably stick with the silver ZX series. MS series is overpriced due to rarity, ZPA series require rebuilds.
rockford - i've owned several era's of RF amps and currently have a matching set of 200a2, 400a4, 800a2 waiting for a purpose. lots of power and a different sound than the xtant or PG (more distortion but in a good way). best to have external crossovers for more flexibility. i like RF amps in most series, very reliable.
PPI - the older PC series was a favorite of mine if you can pick up some PC2150's or similar. I've had the ART series as well and they are neat but prone to noise, IIRC. limited crossover sections.
Soundstream - the older Reference series is a nice choice for an OS build. solid features. i've owned a few but didn't use them. I like matching sets and never had a set.
Alpine - the 1996 MRV series is an excellent amp choice. A MRV-F405, MRV-F505, and MRV-1505 are all Japan made with awesome internals, close to F#1. I ran these for a bit with the Hertz Mille's and they did awesome. I still have the F405. Active capable amps with flexible crossovers.
Orion - i had a few but didn't use them for long. the NT series always fascinated me. Huge amps with plenty of power but require external crossovers and control. Expensive and sought after - a solid investment.
with OS amps you're looking for a 4 ohm SVC sub or one that can be wired to 4 ohm mono.