@Proton Lenny et al
What if you run a split signal with one channel going into the combination filter/comp/chorus (in that order, IMO),
and the other channel going into the Iron Ether FM?
Mix 'em back together and you'd have the attack at the front of the note beefed up by the FM sound on the note's decay.
@EduUY : What you get greatly depends on not just your needs, but your playing style.
OCTAVER
For the
Boss OC-2 sounds great and is a standard, but analogue one-note-at-a-time octave-down and only available used.
The monophonic
COG T-16 is smaller than the OC-2 and sounds great and can be ordered new.
If you need octave up and polyphony, digital is your only course: one of the
EHX POGs OR
much smaller and cheaper and sounds nearly identical to the POG:
Mooer Tender Octaver.
For adding synthiness to lines I'd go for the polyphonic digital octavers 'cause of the octave up, but that's not to say the monophonic analogue can't get synthy, it's just a different flavour and both are delicious. I've got the Tender Octaver for digital and Red Witch is handling my analogue octave down.
ENVELOPE FILTER
A big ol' bad bag o' worms, this is. What you like the sound of may not be what works for you, but you'll hopefully find a sound that you like and is compatible with your playing.
Buy/Borrow/rent and then flip/return/trade until you find something your ears and fingers like.
I'm still relying on the filter(s) in my M5 and hoping to hit upon a dedicated filter I like.
SYNTH BASS
— Check TB thread
Synth Bass Pedal (some great suggestions on Filters here, too)
Panda Future Impact / Akai Deep Impact = pricey, but the Panda FI is powerful and IMO a much better purchase than its Akai grandfather the DI which has less features is less reliable and still commands a hefty sum of monies...
Bananana Matryoshka — small, fun but no presets/MIDI.
Iron Ether Fmeron — as previously mentioned in the thread, an FM synth engine.
Zoom MS-60B — not a true Synth pedal, but with four simultaneous effects built in, some people are getting some very synthy tones from it.
There are other synth pedals such as the RedWitch Synthotron, but I heard it's better with guitar than bass — the ones above are what I'd choose and in that order; other older synth pedals will have to be bought used, be hard to find, may not be reliable.