First time posting here, but I figured this would be the place to get some help.

I am currently in the process of designing my playback and amplification system for my open baffle DIY speakers. My current audio chain is:
Sony TTS 3000a Turntable with AT 440 MLa cartridge-> VLPS II phono preamp -> Decware CSP2+ preamp -> MiniDSP to handle crossover and some filtering -> 300b mono blocks for my full range (Mark Audio Alpair 10.3s in u-frame open baffle) and Rotel 1050b SS amp to H-frame open baffle woofers.

I am having some problems with my gain structure and impedance matching I am thinking, because I am unable to get drive I need to the 300b amps.

The MiniDSP input impedance is 20k and the output impedance is 560 ohms.

I have the gain trims on the CSP2+ at about half way, and even then if I turn the pre up to about 12 o'clock, which is reasonably low listening volume, the input to the MiniDSP is being clipped by the preamp. Using the MiniDSP is introducing about 7.5 dB of insertion loss to the 300b, and I fear that It is not giving enough gain to the 300b mono blocks. When I brought the setup to my friend's house and powered his speakers (KEF C35s) with the CSP2+ straight into the 300bs, the sound was much better, and the amps could be loud enough, even on these speakers, which are not as efficient as my MA 10.3.

Is there something that I am doing wrong, or is the headroom available on the MiniDSP just not enough to handle the output of the CSP2+.

Should I measure the input impedance of my 300b amps, could it be that the output impedance of the MiniDSP is too high? Or is the voltage too low?

I suppose the other option is to run the Alpairs full-range and use the sub out to drive my woofers in mono (crossing them over in the MiniDSP). This does not afford me the ability to shaping the high frequency response (which requires some baffle step correction and a notch filter).

I apologize if this is a dumb question, I am just hoping that I could get some insights from the community here. Thank you in advance.

P.S.
I suppose the other option would be to use a different DSP unit -- like the DBX Driverack PA +. On that unit the maximum input line level is +20 dBU, with an input impedance of > 40 kOhm and output impedance of 120 Ohms, with max output of +20 dBU.