When I use my Big Muff with my MXR Carbon Copy an/or EHX Small Stone Nano it suddenly makes the tone go shrill and hissy, but this stops when I put my Blackstar HT-DUAL between them (I've also tried my Boss CH-1 in the HT's place- doesn't stop it).

However, I'm looking to sell my HT-DUAL soon and it's taking up a lot of my pedalboard space to just be a buffer... So, what's this problem and what can I do? My board is:

Boss TU-2 > Crowther Audio Prunes & Custard > EHX Big Muff USA > [Blackstar HT-DUAL] > EH Small Stone Nano > Boss CH-1 > MXR Carbon Copy
I've got a Small Stone, and have some tone problems when too many effects are running. I usually check the levels that I'm working with and it seems to fix the problem. Are you running the Delay and Phaser out of the loop, and the Fuzz in the main chain?
1981 Gibson Les Paul Custom (Black Beauty)
1980 Marshall JMP 2204
Yeah, I'm not looping anything- it's all in one chain before the amp. Is that the problem?