If you're using the Sig Pros with the WA6SE the tubes make a huge difference in the amount of bass you hear - try to get your hands on some 6FD7s they are the best tubes for bass - with those tubes I never thought the Sig Pros bass was lax, on the other hand with the 6DR7s or 6EW7s they can sound very empty.. This is of course with my Sophia Princess (which also is prob the best rectifier for bass lovers). Like Noxa i didnt find them very comfortable one of the main reasons for the sell also for me the soundstage was lacking as well - otherwise one of the better headphones out there for the price.
I've been using the sop princess and I just ordered a few more pairs of tubes. Now is the time to try tube rolling to see where that gets me. Other than the less than stellar bass the sigs are fantastic but i bought them mainly for travel and i think a noisy environment would further diminish the bass. I'm gonna stick it out with them for a while and see how it goes. Unless you wanna trade for your TH900s?