I bought this Bass Tube by Schlagwerk for a Tycoon Percussion Cajon.
I have to say that it does the job. From the first moment it's fitted you can hear the difference right away; the bass frequencies are much better, and it helps in separating snare and bass sound.
The problem i had is related to the fitting. I didn't mount it on a Schlagwerk Cajon, but i read that it could fit on any hole of approximately 12 cm.
My Cajon is 12 cm exactly, but still, the Tube wasn't staying on the hole at all, i had to apply some glue anyway.