Mac, regarding your question I’ll just second the good answers provided by Bill (Grannyring) and Kenny.
Regarding 2.83 volts vs. 1 watt, power into a resistive load equals volts squared divided by ohms. So 2.83 volts into 8 ohms is 1 watt, but 2.83 volts into 4 ohms is 2 watts. And since a power ratio of 2:1 corresponds to a 3 db difference, if the SPL of a 4 ohm speaker is specified on the basis of 2.83 volts (as is the case more often than not), 3 db has to be subtracted from that number to get SPL on a per watt basis.
The rationale for specifying on the basis of 2.83 volts is presumably that most solid state amps can supply considerably more power into 4 ohms than into 8 ohms, in some cases twice as much (i.e., 3 db more). However most tube amps cannot do that, even if they provide a 4 ohm tap.
Cal, I’ll mention another issue concerning passive biamping, which applies even in the more typical case of a speaker having a much higher crossover point between bass and mid/hi than 110 Hz, and which electrically filters low frequencies out of the signal provided to the mid-range drivers (which the PRE does not). If the amp powering the bass driver is much more powerful than the amp powering the mid/hi drivers, chances are that a good deal of the power capability of the bass amp won’t be able to be utilized. While a passive biamp arrangement reduces the amount of current and power that has to be supplied by the amps, both amps still have to output a voltage corresponding to the full frequency range of the signal. So turning up the volume control high enough to utilize most of the power capability of the higher powered amp would most likely drive a significantly lower powered amp into clipping, resulting in severe distortion.
That fact seems to often be overlooked, when people consider passively biamping a high powered amp (often a solid state amp) in conjunction with a relatively low powered amp (often a tube amp). Active biamping, with an electronic crossover "ahead" of the amps, avoids that problem, but potentially has sonic issues of its own.
Best regards,
-- Al
