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.

Make sure that your CD player can output at least 4 volts (double the Phillips red book CD standard), because the Mc402s sensitivity for full output swing is 4 V.
I have had great results with Wadia and Audio Aero Capitole CD players.
A regular 2 V output CD player will cripple your Mc 402 to 200W/channel.
If you match properly, it is a killer combination, one less box to worry about, one less power cord and interconnect.
Good luck

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