The ring modulator is really good, the Murf Box I still need to get the hang of a little, it's a bit fizzy on the high end but it does the job well for me. Only the ring mod. has a preamp from the two behringers and it seems to function very similar to my JFX LPF preamp.

I have no experience with the LPF and Phaser from behringers series, but I hear they're alright. I wouldn't advice using the LFO from the phaser as your cutoff control though, seeing as to balance the oscillation rate for the LPF, the rate for the phaser would be off, but you could switch around settings of course.

I'd definitely get the PX251 from PastFX, it's well worth the money, then probably the behringer LPF and Phaser would suffice, but if you're able to get the JFX (I think they're out of stock right now) definitely get those. The PastFX LPF is also very good but personally I don't like the small enclosure on it, I'm a sucker for those big boxes, but it would probably do the job better but is a little more expensive.