The S1 Cafe - Helping Your Espresso Dreams Become Reality

A bit of background:

I got my Vivaldi II from Chris' back and 2009, paired w/ a Rocky. Worked great, pretty easy to use a dial in.

I moved away for ~2 years, sadly leaving my S1 behind. When I got back I really struggled to get good shots. Either too fast, or spitting / channeling (a big mess). I could see lots of channels. I got switched to a Niche grinder (wanted to be able to switch beans quickly to do other styles), which provided finer control, and I think it helped, but not much.

I'm using the double basket it came with.

I've been tracking my dose/grind/time for a while, and can't seem to hit it, eg:
(dose, grind, time)
15.5g / 12+ / channels
15.5g / 13 / 10s
15.6g 12.5 / 16s, spitting
16g / 13 / 10s
15.2g 12.5 / 12s
15.4g / 12.5 / 10s, spitting
15.4g / 12.7 / 10s, spitting
15.4g / 12.5 / 13s
15.4g / 12 / 12s
15.4g / 12.5 / 10s

I've also tried going up towards 17g and 15 grind without much help.

etc. I use a distribution tool to level, and a tamper with a rim for a consistent final level. I've adjusted the pre-infusion to 6s, and even tried tweaking the water pressure (plumbed in). I've read lots of posts here about the S1, and the topic in general other places, and I'm just out of things to try.

Any ideas?

One followup (Can't edit): I was seeing the pressure go past 10bar so I adjusted the control on that so it's more at 8bar (rising a bit during the shot), that dealt with most of the channeling (this was before I started writing down all the shots)