Seems like either the preamp is got too much output or the power amp input sensitivity - not a issue of wattage. The industry standard for single ended (RCA) wiring is 2.0 volts on each (preamp rated output and power amp input sensitivity) so they play nice in any combination. Most power amps are also rated around 26 dB of gain (regardless of power rating) but some are designed to work with passive preamps so have closer to 32 dB of gain.
Ideally you’d have a more usable range of volume control and the typical setting would be around 9 - 3 on the dial, or half way on a digital display.
Back in my hair-shirt audio days I used stepped attenuators versus a preamp that were connected between interconnects and the back of my mono-blocks. Good sound, but inconvenient.