You should never have a full seal with earbuds, not only is it dangerous to your ears but also completely ruins the sound as the driver membrane can not take that pressure given that all of earbuds are designed for compensation in mind (for some of the lower frequencies leaking).
For me, putting buds sideways makes them 10 times better - removes nasty treble peaks (that are present with all buds to some extent if I wear them normally), brings the bass up quite a bit but it doesn't reduce sound stage nor the open nature of sound. I also don't use any foams with MX shell and I need them for RX-1 which is a bit smaller and doesn't properly sit in my ear sideways without foams.
What ends up happening is this - only a couple of holes in MX front shell end up firing sound into surroundings, some of them are closed by my ear and most of them are directly shooting into my canal - compared to wearing them regularly where essentially most of the bass gets lost regardless of foams and some of the holes are closed, some shooting sideways in my ear yet most of that sound can't even enter my canal as the bud is blocking it and none of the holes are firing in my canal at all. I never had fitting issues with dozens of IEMs so my canal has quite the average position.