Subsonic filter or not ?

It depends on the tuning frequency.

Don't play notes that are well below the tuning frequency and you'll be alright.

Yeah, subsonic filters become a lot more important with high tuned SPL types enclosures, but if it's tuned low it should not be needed.

I ran an RE SE12 in a ported enclosure tuned to 32hz and did not use or need a subsonic filter. You have to be somewhat under the tuning frequency before the woofer completely unloads in the enclosure. It's not like an enclosure tuned to 30hz, unloads at 29hz, it gets progressively worse as frequency decreases below the tuning frequency.

If you have an enclosure tuned to 32hz, I wouldn't start playing 20hz test tones very loud.

So, with that said, tune the box low enough 28-33hz or so and listen to music instead of 20hz test tones and you won't need the subsonic filter.

That sounds good to me!
I don't listen to test tones ! only music, and I will try to tune low.
Next time I buy a sub amp, I will try to buy one with a subsonic filter!

I think that a SSF is equally important for the amp. Why make it struggle and try to reproduce those super low frequencies when you won't be replaying them to begin with? The less your amp struggles, the better things will sound and the longer your gear will last. I do agree with everyone else about the SSF as safety for a driver in a ported config as well as tuning the port low 28-30Hz.