Bi amping with two different pairs of monoblocks

Bi-amping isn’t something you do half-baked.  Y cables will attenuate the signal somewhat, but the real point has been made above.  If you are going to biamp, you should be using an active crossover between your preamp and the power amps. That is the correct "Y cable." 

I have tried the method above and it may sound cool at first but you will soon realize the imbalance in power between the monoblock pairs unless they are identical.  Good crossovers have gain adjustment to get it right. 

I would look for a good analog crossover. Marchand makes some decent ones without being too budget-crippling and Phil will answer your questions directly. 

I should also say that bi-amping really only makes sense if you can bypass the crossover in the speakers. 

     As others have pointed out, no 2 amplifiers, especially if from different manufacturers, will produce the level of sound from the same preamplifier input signal,

Ideally , Bi-amping should have identical pair of stereoamplifiers or 4 mono-blocks, as most audiophiles know. different amplifiers or monoblocks , can work as well , if you insert volume control just before the more powerful amplifier. this volume control can be passive volume control using resistors & you can find this in DIY Audiophile oulets, e,g.  Hificollective, Uk or similar. or, Transformer based Volume attenuator, which is not dissimlar from some passive preamps , which is based on transformer , but, this one has only 1 input & 1 output. I have this setup in one of my systems. T/T; Rui Borges , Arm; Safir9, Cartridge; Goldfinger, PhonoStage; The master groove Signature, Preamplifier; Wafac PRT modified , 4 mono blocks driving Vivid Giya 2. Valve Amp, Audio Music 833 Ultima Siver transformers Monos , deliver around 150 Watt to High & mid part of the speakers straight from preamp to the monoblocks using 3 meter IC. The bottom end Using SS Acoustic Art III MKII, which deliver over 800 watt. IC from the premp is 2 meter to volume attenuator, which is specially made silver transformer in pretty box by Audio Music ( china) , then 1 meter IC from the volume control box to the SS Mono. The sound is simply glorious as you match the output of the stronger Amplifier to the other one through volume control. you don`t have to make a special volume control box. Any good passive premp will do ( Resistor or transformer based) Obviously, better quality will be more desirable. chose the one with one input preferably. Happy listening

Ideally , Bi-amping should have identical pair of stereoamplifiers or 4 mono-blocks, as most audiophiles know. different amplifiers or monoblocks , can work as well , if you insert volume control just before the more powerful amplifier. this volume control can be passive volume control using resistors & you can find this in DIY Audiophile oulets, e,g.  Hificollective, Uk or similar. or, Transformer based Volume attenuator, which is not dissimlar from some passive preamps , which is based on transformer , but, this one has only 1 input & 1 output. I have this setup in one of my systems. T/T; Rui Borges , Arm; Safir9, Cartridge; Goldfinger, PhonoStage; The master groove Signature, Preamplifier; Wafac PRT modified , 4 mono blocks driving Vivid Giya 2. Valve Amp, Audio Music 833 Ultima Siver transformers Monos , deliver around 150 Watt to High & mid part of the speakers straight from preamp to the monoblocks using 3 meter IC. The bottom end Using SS Acoustic Art III MKII, which deliver over 800 watt. IC from the premp is 2 meter to volume attenuator, which is specially made silver transformer in pretty box by Audio Music ( china) , then 1 meter IC from the volume control box to the SS Mono. The sound is simply glorious as you match the output of the stronger Amplifier to the other one through volume control. you don`t have to make a special volume control box. Any good passive premp will do ( Resistor or transformer based) Obviously, better quality will be more desirable. chose the one with one input preferably. Happy listening