The vent only contributes output below Fb, the subs tuning frequency. While its true all vents contribute some noise, the ports on the ep350v3, ep500 and ep600 are designed to be very quiet. They contribute far less resonance than smaller round or slotted ports found on most other subs with this alignment. Its an important aspect of overall sound quality when comparing vented subs.
Bloated thick bass is not characteristic of the ep600. In my HT, listeners usually describe the 600 as more natural and full of textures. Bloated sound from a high quality sub arises when the mains/sub are not in phase, and/or not blended well at the crossover point and/or room acoustical problems at those frequencies and corresponding room nodes.
40 hz is too low with the 150s and qs8s. If you have a flexible bass management system which allows individual crossovers for speakers I would set those at 90hz and certainly no lower than 80hz. Also setting the crossover too low puts more burdens on low powered receiver amps which are overtaxed to begin with in most HT systems.