HOW DOES MC402 SOUND WITHOUT PREAMP?

Face, a passive preamp is not a good match for the Mc402, because the Mc402 requires 4 Volts to achieve full power.
If one connects a regular (2 V out) CD player through a passive preamp to an Mc402, the amp will output only half its power rating, a waste of time and money.
The correct setup with an Mc402 is a CDP with an internal preamp and high voltage output like Wadia / Audio Aero or lacking the former, an active preamp with gain.

It's 2V unbalanced and 4V balanced. My SD transporter puts out 2V unbalanced and 3V balanced. I never really had a problem with the input. I bought a c2300 preamp and hooked it up. I could turn the volume up loud enough to start seeing the power-limiting lights kick in but it never seemed to really get much louder than without the preamp. My speakers are inefficient (86 dB) so they can pretty much absorb all the 402 can throw at them. I decided I didn't need the pre-amp after all. The DAC at full volume is more than loud enough and I never listen to it all the way up. Even at -5 to -10 dB it's really too loud to the point that it makes my head and neck tense up. I wouldn't necessarily worry about getting that last volt or two out of the preamp. In my experience it's unnecessary.

Wireless, you are correct regarding maximum loudness in your system.
IMHO, loudness and dynamics are different qualities of a sound system.
Loudness is about room sound pressure level (SPL), dynamics is preserving intact the webb and flow of music from pianissimo to fortissimo. The Mc 402 excells at dynamics, for example a piano recital can sound whispery one moment and thunderous the next moment.
I for one realized how limited were my previous amplifiers in terms of dynamics.

There is a unwritten rule-of-thumb for dynamics: when assembling a system, choose components with enough gain to reach maximum power, then leave a safety margin of at least 6 db.
You will be amazed at the increased slam and dynamics of a system with 6db or more of gain to spare. Also, extra gain helps with the odd lute concerto or CDs not normalized (during mastering) to achieve maximum loudness at zero db.
Enjoy the music