I'd do the sws8's and amp before adding the ms8. The ms8 is not very powerful and the underseats are 4 ohm so they will get alot less power than stock. With just the underseats amped it should actually sound very good.
2006 BMW 325i Titanium Silver
Mods- 19" AG M310's hypersilver, LCI xenons, drilled and slotted rotors, OEM rear spoiler, OEM exhaust tips, brushed black interior trim, Viper 5904 alarm + remote start
Audio upgrades- Alpine S920HD, JL XD600/6, Jehnert XE200, Morel Hybrid Ovation II, Audioque 2200D, 2 Alpine SWR 12d4's (forward facing sealed from trunk)