I've got 775 XT brakes on 2 different bikes – one set has really firm levers and has great feel, but the other remains a bit squishy regardless of all my bleeding attempts. I've tried bleeding top down & bottom up, drained and refilled them etc but neither front or rear will reach the same firmness as the other set – which I also built up myself & bled really easily.

Pads etc aren't a factor as the squishyness is evident with the yellow bleed block in place. There's no apparent leak from the squishy set, and the adjusters on the levers are set the same. I'm thinking the next step might be to swap one of the levers over between the 2 sets and see what that does, but any other bright ideas?

Don't start messing with levers. You'll just end up with 2 squishy sets!

Shimano brakes are notorious for being hard to bleed – I guess that the internal shape means that air bubbles get trapped easily. I've solved this on a mates bike (similarly frustrated with the sponginess despite numerous bleeds) by moving the caliper to lots of different angles, tapping with a spanner (to try and dislodge any bubbles), and then doing similar at the lever end. It did work but it was a right faff…

Clubber – I've tried bleeding these things in so many positions I could write the Shimano Kama Sutra! Even tried pumping the pistons out a bit then pushing them back with the bleed valve open to pursuade any tricky bubbles out, but still no luck.

It's not just the position – it's the tapping that really gets things moving – it did leave marks on the caliper mind (I used a spanner) but the owner was happy enough with that to get them working right.

dont know about xt, but i had some good success with my formula oros, by once id bled them, close the lever, and leave it cable tied down. and then pull on the caliper syringe until little bubbles come out. keep doing this. pull, let go, pull, let go etc until no more bubbles.
repeat at lever end. then put all the bleed screws back in etc and see how that feels. basically your pulling air out from individual ends with the system clamped shut in the middle, rather than keep pushing them back through the system when you do the push me/pull me thing with 2 syringes.

like i say, it works with formula, but ive no experience of how you bleed xt.

i bleed mine bottom up from scratch with the caliper in a vice (gently) so that the bleed nipple is the bottom most point, and the hose extends upwards the whole time to where the lever is. I then leave em over night with a little pressure on the lever, and tap em real good after. Listen to the caliper as you pull the lever of the squishy one, maybe no oil, or not much oil is escaping but a 'squelch' will tell you if they're b*ggered.

Are you bleeding them with a spacer between the pistons? If so, try a thinner spacer. I had a similar problem with my new XTRS while using the suppled plastic spacer -everything was hunk y dory, with all air seemingly out. When I took out the spacer, fitted the pads and then the wheels, the levers far too much travel and were pretty ineffective. I eventually retried with a thinner (home made) spacer and all was well at the 1st attempt.