Duh-Forgetting Hifi Basics 101-crossovers

OK.Sold hi-end for 6 years.Said a lot of stupid stuff here maybe occasionally something with some salience (yeah yeah -don't replay "Don't be on it chazz-I can imagine).But now spacing on bi-amp basics.Know if you have mono conversion switch you can double power vertically.And remember that horizontally you can get 150 wpc summed power from two amps.Also know that one needs level control if using two different amp putting out n different amounts of current and this is feature of many crossovers.But if using identical amps and hooking one amp to tweeter posts and one to bass posts (or tri-amping) and removing jumper why is it that one needs x-over to to limit full range signal being sent from being sent to either set of drivers.
Isn't the the crossover still in the signal path creating it's high low pass?And if I need a means to low/high pass how can I do this as cheaply as possible.Amps are either two Denon 1500's (150 wpc) or PS Audio Delta's at 100 wpc.Neither can be converted to mono for vertical bi-amping and think though this is done many times for greater power yet I think better results are are gotten through going horizontal.
Cheapest and lest efficient (power robbing) wold be passive in line filters between pre and amps.But this set up is not worth cost of a Bryston 10B or a Marchand tube unit.
So 1) why with same amps do I need a x-over and 2) what's cheapest way to hook up my amps and two ways yet retains acceptable sound quality i.e. best cost to perfromace ratio.It's been a long time since I have thought about issue and through i was explained to me as a kid before I worked in shop (where we rarely set up bi-amp set ups.This is embarrassing it's so basic so be gentle as I struggle with early on set Alzheimers.
If you're "passive bi-amping" with the speaker's cross-overs you're not going to have a significant power increase.

Both amplifiers are seeing the same signal. Their power supplies might sag a little less when feeding only one driver, although a couple volts out of 40 (~100W into 8 Ohm amp without the excess to account for diode drop) is only going to get you .5dB more before the amps clip at the same point. That may not even be audible and isn't going to sound like a volume increase.

If you removed the passive cross-overs and used an active XO you'd get a gain. If I had an 8Ohm speaker with 2.83VRMS signals for woofer and tweeter, with a passive cross-over I'd need an amp which could hit 8V peak rated at 4W into 8 Ohms. With an active cross-over I'd have two amps each producing 2.83 VRMS,4V peak,and rated at 1W into 8Ohms. Since the musical power spectrum favors low frequencies, it's not 2 x 100W = 400W. 120W + 80W active = 400W passive in an average 2-way.

Replacing the stock cross-overs with an off-the shelf unit is VERY unlikely to work. Most speaker designs achieve their cross-over slopes by combining the speakers' natural roll-off with a lesser electrical roll-off. Many have shelving filters for baffle-step correction, notch filters to tame resonant peaks, etc.

If you want to actively bi-amp, you'd need to measure the electrical transfer functions of the passive cross-overs, bypass them, and duplicate them using a DSP (Behringer has one for ~$300) or analog active cross-over which will produce the required slopes (you could populate Siegfried Linkwitz's 3-way ASP or 2-way + power amp Pluto boards for somewhat more).
Wasn't sure about he physics of i all but ha been erroneously informed that getting active x-over with correct slope that sounded me right would get 1 1/2 of power total.I have a 80's $1200 (now $300-$400) amp and thought it might be good for a 2 way monitor.Every time I have heard bi-amped 2-way's they sounded better making woofer less strained and less distortion than single amp set ups'.Have looked for cheap x-over in google but most are sub area.But I realize while people go vertical if they have switch.Still think my buddy who had two Denon 1500 low current mid-fi's with a the small crossover from Maggie.Worked with III';s but when I found him 3.6 a sinle Bryston 4 BST blew it away.That set up deserves the qoB (and good sub) and another 4 ST o a pair of 7 mono's or 14 B ST's .
Thanks again
But if using identical amps and hooking one amp to tweeter posts and one to bass posts (or tri-amping) and removing jumper why is it that one needs x-over to to limit full range signal being sent from being sent to either set of drivers.

There is no need for a crossover in this situation assuming the speakers have an internal crossover. Passive biamping as you describe will have no effect on the amount of distortion the woofer produces, since that's a function of the driver/cabinet and not the amp.

Even with the internal passive crossovers intact, one could employ an active (or passive) crossover before the amps. A smaller amp for the tweeter could then be used.

Just an FYI, Rane makes affordable active crossovers.