Sadly a week ago one of my earpieces stopped working so I am waiting on a replacement.
The five days that I used them I seem to settle on the Bass=, Treble+ which for me as "epithetless" described the bass is present when it should be and with a nice amount of body but I agree a little more smack or impact would not go a miss.
Moving on, as others have tried I also experimented with a few alternate tips finally settling on q-Jay silicon tips which I find more comfortable than the Sleek flanges but also the silicon is slightly thicker which for me improves the isolation and tightens up the bass a little. I have not tried the Shure Olives that others seem to like but I have ordered some so hopefully they will arrive around the same time as my replacement earpiece.
With my q-Jays and MylarOnes before those I found I got best results by using large tips and creating a seal at the opening of the ear canal however with the Sleeks I decided to try using medium tips which take a little more adjustment but I find I can get a good seal around 4-5mm deeper where my ear canal narrows and tends to slope upwards.
Although there is little difference isolation wise this does create a noticeable changed in the bass characteristics with the Sleeks becoming warmer and the bass more resonant. I think with the slightly deeper insertion of the tip more sound is being conducted through the bone at the entry of the inner ear rather than being absorbed by the fleshier outer ear. With the added warmth I also moved to the Treble++ lifting the high end a little.
Whilst waiting for my replacement I have also fashioned an Extended Base Port which increases the length of the bass chamber around 4mm. With only one ear piece for testing it is hard to tell how much change the extended port makes but even with a mono test I can already notice subtle changes in extension, separation and definition in the mids and low end.
Personally I am having a lot of fun with my Sleeks and enjoy reading what others are doing in the quest for their Ultimate Sleek Sweet Spot.