Triple mica refers to the number of mica spacers inside the tube (3). Many tubes have only 2 or 3, triple mica 5751 are renouned for being good across many brands.
The Sylvania's let a detailed treblel and a punchy bass shine through along with a beautiful midrange. They are forward sounding, very good compliment for my MG Head/HD600's. Most of the tubes I've tried have either been bassy, soft on top and not very detailed, or the opposite: dry, lean on the bass, and possibly hard on top. The GE 5751 Triple Mica is good, but I found it a bit hard in the treble, not as detailed as the Sylvania's and with a less prominent midrange. The Sylvania can sound delicate, detailed and yet punchy, full and not at all fatiguing at the same time, something I've not found in any other tubes in my MG Head.
If you haven't heard Nick Dangerous's MG Head Tube Rolling page,
take a look. It primarily discusses 12AX7 type tubes.
Joe's Tube Lore is another excellent reference for 12AX7 tubes (very similar to Nick's, but more in depth, and both covering a few different tubes).
-dd3mon