I too agree with ZD. I would add, with respect to the passive biamp configuration you are proposing, that in most cases it would not be correct to match the sensitivities of the two amps. What should be matched is their gain, the relation between voltage out and voltage in.
Sensitivity is the input voltage required to drive the amp to some power level, usually its maximum rated power. Since the max power ratings of the two amps would probably be different, if sensitivities were matched the same input voltage would result in different output voltages from them, and hence different output powers for a given load impedance.
Power amp gains are often unspecified, but are commonly indicated in measurements provided in reviews, such as those in Stereophile. Also, gain can be approximately calculated from the sensitivity and max power capability specs of an amp, as I described here
Finally, I see that your present amp is rated at 8 watts. A point to be aware of is that if you were to passively biamp that amp with a much more powerful solid state amp, that is somehow properly gain matched, you would not be able to utilize most of the power capability of the solid state amp without driving the lower powered amp into clipping.