To help silence the spacebar, in addition to installing a Silencing Ring on the center switch, I lube the stabilizer and switch rails with Superlube 51010 oil and apply extra pads under the space bar. Although the HHKB already has spacebar shock absorbers, I add squares of self-adhesive rubber (like the anti-skid strips on the underside of keyboards) on top of the shock absorbers. For RF and NT boards, I use thicker rubber so that it makes contact with the underside of the spacebar. It can also help to put some rubber foam strips inside the spacebar to deaden its sound, but don't overdo it and make the spacebar too heavy or this will affect the return stroke.