Boba U4's have softer cushioning on the bottom and a quieter slide over the tactile bump, making it noticeably quieter overall. Silent Bluish White are less mushy, so when you bottom out a stronger shock will be transferred to the plate and the keyboard build and materials will have a lot to say about volume and how that shock sounds.

In my quest to try out most sound-dampened switches on the market, what I'm really looking for is a switch with low-volume (quiet, not necessarily silent) that still sounds good. In this regard Boba U4 doesn't score well because I think the sound it makes is a bit thin and flimsy. Put them in a well dampened keyboard however and you have a very silent and enjoyable feel which no other high-tactile switch I have can match so far, including TTC Bluish White, Haimu Whisper, Durock Blue Macaw and Kailh Deep Sea Brown.

Would love to try the Outemu Peach and Lime switches. I use both U4 and Midnight Pro (the very lightly tactile one) and I enjoy them both despite the latter being more noisy, it makes a very soothing marbly sound on in my kbd67 lite (PC plate, foams).

U4 was the quietest switch I had tried until Mode Anthracite. They initially felt too heavy, but they're growing on me. I'm so pleased that mode even bothered making a silent switch and they bring something new. Less tactile than U4 but still noticeable.

To make a quiet keyboard you have to start with the switches, they are the source of vibrations to begin with. But the plate material and case have a lot to say what happens to the sound that is left. A metal plate in an open case is certainly going to accentuate any spring ping, leaf making contacts and plastic scratching.