Can’t speak for anyone else but for me ITB Heat alternatives are -
Fabfilter Saturn
Sly-Fi Kaya
U-He Satin
Klanghelm SDRR
D16 Decimort 2 (bitcrusher so kinda different but equally useful for mojo/character)
Slate Virtual Tape
Soundtoys Decapitator
Otium Sonitex
These cover pretty much every shade of dirt/tape/mojo I need and sound legit. I normally dont need to use ITB filters so I can’t offer any recommendations there (tho Soundtoys FilterFreak 2 normally sounds pretty good if I do ITB filter).
Had a few plays now with a friend’s Heat at his studio. It’s a great box. On my list. But the above plugins sound just as good/better to me for ITB work and multiple instance has obvious benefits. Heat’s very attractive still tho for sound design/hands on/playing shows without computer.
If the OP is just after subtle saturation/vibe they might be happy with a Deco? Save themselves some £ if distortion isn’t really their thing… Heat has a lot of uses tho. Env follower, Lfo distortion/filter etc etc.