I am using a RM Italy HLA 300+ amplifier mobile and it works well and I get great signal reports most of the time. This model does have filters for each band automatically switched. My question would a stiffening capacitor on the amplifier power input improve peak power. The voltage at the amplifier would be 14.1 volts but drops to 13 volts on ssb voice peaks. 4 gage wire direct to the battery powers the Ft891 and the amplifier. My antenna is a Tarheel 75A with a 30 inch rod and a capacity hat on top. This works better and uses less of the coil on any band compared with the 6' whip.
Disclaimer: I have, and can, operate legal limit mobile. I've also set up mobiles bigger.
You need to find out what the voltage is at the alternator.
You need to find out what type of charging system your car has. Depending on the vehicle charging system you may or may not want to run a beefed up alternator wire.
Case in point. My old 77 GMC 4x4. Had a #10 wire from the alternator to the battery. I would drop serious voltage anything over 100 watts.
Changed that one wire out and added a wire from the negative terminal of the battery to the body and frame, and one to the alternator mount.
Solid 13.8 volts, keyed down. 14.4ish key up (it would run near 14 volts constant without the station running). That was a 4 x 2879 amplifier and IIRC a 70A alternator (or 65 maybe?

).
OTOH, 16 x 2879s, 2 x 165A leece neville alternators, a deep cycle and a standard car battery run just fine at 14.4 volts, key down.
My point is, if the voltage is dropping at the battery, then nothing you do at the amplifier is going to help. If the vehicle electric system isn't up to the task of your amplifier (if it's doing 300 watts, it's pulling 600, that's 50ish amps peak, itself) and if the alternator isn't up to the task, if the wiring from the alternator to the battery group isn't good enough.... Etc.
Find the voltage drop at the car battery. If it's dropping there, fix that problem first. But make sure you have a charging system where you can just throw larger wire at it, because not every system is capable of doing that.
--Shane
WP2ASS / ex KD6VXI