I have an SB-15 with a KAK tube and I want to try something else. For a short pdw style weapon I actually would prefer to have the sb-15 closer towards the receiver than the KAK allows, if it were a normal stock on a full sized carbine the 2-3 position would be a bit more optimal.

So I am going to buy a replacement buffer tube, since the KAK seems pretty heavy (..?) I was hoping for a recommendation of a light tube that works with the sb-15

Well, the KAK Super SIG says it's 6.2oz, the Phase 5 Pistol works out to 6.94oz (weight might include foam pad though, unclear), and the PWS EBT Pistol claims 5.4oz (again unclear if includes foam pad).

I like the PWS EBT Pistol for its QD sling attachments, extended lip, and castle-nut-free design - but note that the stub on the SB15 hits the PWS as-is.  This adds ~1/2" to the length - which some folks might like, and others might choose to X-acto off the SB15 stub...

Of these, the PWS looks lightest - but I think it's also the priciest at close to $130  I didn't weigh mine before installing, but I'm happy with it so far.

HTH,
Richard