Bi amping using Devialet 120 and Cayin A88T MKII (Passive)

[Drifter]

pass through should also work, I never used it.
For a 3-way active crossover you need either 3 Devialets or 2 Devialets and one power amp.

Here I'll address @Drifter's point:

The attenuating problem you mentioned seems only applying to Method 2 but I think Method 1 should be scrutinized under the same lenses too: does Method 1 overcome this attenuating problem by connecting pre-out on Devialet to pre-in on Cayin?

I learnt that the key condition to do bi-amping is to match the gains of two amps (is it the attenuation problem we're addressing here?). But how to do this? How to find the gains of two amps (I cannot find it in the manual of Devialet and Cayin). Or I should just match the output powers (Devialet 120W; Cayin 45w/25w)?

OK, this question may seem stupid but, with two integrated amps, why cannot I just connect the same audio source to the two integrated amps using RCA splitter cables and then two integrated amps to HF and LF posts of speakers? In this way can I address the gain mismatch or attenuation problem by adjusting the volume controls on two integrated amps respectively until a perfect match of bass and midrange/treble is achieved?

Thank you~

You will have the same issue (level matching the 2 amps) for both option 1 and option 2. It is not just the very different power outputs that plays a part (120W versus 45W) but also  many other factors that will cause big difference in volume between two amps. You may get 2 power amps from different manufacturers with exactly the same same power output rating, through they result in different volume levels when driving the same set of speakers at the same volume knob setting on the pre-amp. 

You will have to attenuate the Devialet down (assuming that its higher output power will result in more volume than the Cayin at the same volume dial setting) and you will have to measure the levels with some or other db meter to ensure they are exactly equal. Otherwise you will get louder mid and highs than bass when playing a track.

Your next plan (in green) is an issue as you will have 2 integrated amps with 2 volume knobs. If you want to increase the volume, you will have to turn both amps up. How will you know how much to increase each to still have the exact same volume level from both amps?