My present power amp is X1000.5, I think it is enough for many speaker.
Wow! That's for sure!
I'd just leave well enough alone, unless you feel the need to upgrade the speakers or something else in the system.
Even though all of the Pass amps are spec'd as having the same 26 db of gain, so you presumably wouldn't have a gain matching issue if you biamped, I suspect that biamping those amps with a different (and presumably lower powered) pair of Pass monoblocks stands a greater chance of adversely affecting sonics than of providing a benefit, as stated above and in the link I provided.
And adding two more X1000.5's, besides costing a great deal of money without necessarily providing any benefit, raises concerns about simply providing them all with enough AC power. You'd undoubtedly have to power each pair from different circuit breakers and different dedicated AC lines. Which in turn would create some risk of introducing ground loop issues into the system, although the fact that the components (and I presume their interconnections) are balanced would minimize that risk.
Sometimes the best thing to do is nothing. Or as the saying goes, "if it ain't broke, don't fix it."
Regards,
-- Al
