The power issue is all about driver sensitivity. Your drivers are 3dB LESS efficient than HiFi or L7 drivers (per 2.83 VOLT). Your drivers are 87dB/1w/1m@4ohm which equals 90dB/1m @ 2.83v. The HiFi/L7 are 87dB/1w/1m@2ohm which = 93dB@2.83v. The Rainbow are the same 93dB@2.83v (90dB/w@4ohm).
Since the MS-8 does NOT have a impedance sensing switched power supply, you need only to figure out what the sensitivity of a given set of speakers is @ 2.83v...it doesn't actually matter whether the driver is 2ohm or 4ohm, the MS-8 will put more watts into a 2ohm but not any more volts. Basically the MS-8 is equally loud on:
Rainbow SL100 4ohm speaker (90/1w/1m) on ~18w of MS-8 power
OEM L7 or HiFi 2ohm speaker (87/1w/1m) on ~30w of MS-8 power
They are the same.
WTB - Used - 1x Blizzak LM-60 225/45/17