As others have noted, the first thing you might want to check out is your room. Rooms modes and nodes are weird and just because you didn't notice a mode or node with one speaker doesn't mean you might not have an issue with a different speaker.

But honestly it might also be the speaker.

The S2EXv2 is a great speaker at its price point, and it has solid bass response for a bookshelf speaker, but that last bit "for a bookshelf speaker" matters. It's still a ported bookshelf speaker with a 6" driver. I'm a bassist and with the EXs I run a sub because I miss those 35-45Hz fundamentals that the EXs don't quite deliver.

So, yes, the EX probably is bass-shy compared to a transmission line floorstander like the one you replaced. It's not hard to believe that the Sierra wins in terms of treble and midrange performance but does not compete on bass. IIRC PMC is particularly proud of their transmission line design so you may be comparing the best thing about the old speaker to an area where the EX is good for its class but not great in an absolute sense.

Incidentally, while I use a sub with my EXs, I listen to the Sierra LXs without a sub and I actually have to EQ the response around 50Hz down because of bad room mode at that frequency. I also didn't need to do that with the EXs. The Sierra LX is the first bookshelf I've ever heard that really feels like a full-range speaker. The EX is a great speaker but it simply does not dig as deep. If you're opposed to a sub I think the LX is the superior speaker.