I had a pair of S6 and had the same problem. The bass output is just too strong and lacks control. This has a negative effect on mid-band clarity. As for the tweeter roll-off, that´s how Dual Concentrics usually sound, especially the Saturn line because it´s the cheapest, and it´s not transparent on the HF. I suggest you try with amps, sources and cables with fast bass, they can help a lot. Inserting the foam plugs will help reducing bass energy and the speaker will sound more controlled, but with less air and smaller soundstage. Bass reflex speakers are designed to work with the ports open so the foams are a solution that partially compromises sound.
The Saturn S8 LR (monitors) are best than the s6. And even better are the System 800, which are studio monitors and have the same drivers, but stiffer cabinets. I presently own one of these and they really are more transparent and fast.
I´m writting from Chile, so if there´s something you don´t understand I apologize. Regards.