It is not the wattage you have to match; it is the sensitivity and gain. That said, you will find it hard to do without introducing a gain control on one or the other amp (whichever has the higher gain). Whatever.
AR S-40 speakers, nothing great. I am somewhat wondering for future applications. I know the Hafler makes a great bass amp, and would love to have tubes for the high end. Am curious what it takes. Ideally I would retain the full power of the Hafler for the bass while introducing tubes for the high-end. More sensitive tweeters sounds like a good solution.
"More sensitive tweeters sounds like a good solution" to what problem?
If the speakers you're biamping are 2-ways with only treble in the top end, you'll need probably only a few Watts to drive them. A Single-Ended Triode (SET and NOT 'set') with single-digit Watts of power would be ideal for that. If the speaker is a 3-way with midrange and treble on the top, then you need lots more power, but probably not as much as you think. I'm driving rather insensitive (86dB, I think) Quad 989s full-range with a pair of 50-Watt Antique Sound Labs Explorer 805 DTs very succesfully in my fairly large (c. 3200 cubic feet) room. Depending greatly on the sensitivity of the speakers, you could be well served with 25 - 100 Watts of tubed power for the MR/treble.
As Kal indicated, you'll need level (volume...gain...) controls on the more-sensitive amp to balance the top and bottom, or an active crossover with such controls.