A single PB10 vs. your 7001's will be a lot closer than you think. Powered woofers in towers are limited by the smallish footprint of the cabinet, but more importantly the placement of the towers. The chances of the placement of the towers at properly listening position for the midrange/highs *and* sub is practically nil.
Even so, what point are you trying to make? You're comparing $1750 towers with a less than $500 sub? Obviously the bass wasn't enough -- even with FOUR towers, you still got the Trinity
Deepest bass from a bookshelf? Not sure if it's considered a bookshelf as its kinda big, but the S-150THX from M&K would get my vote.
For true "bookshelf" size speakers, there are going to be inherent tradeoffs. The cabinet is only so big, and taking 90% of it up with a bass driver is going to have an impact on the midrange/tweeter. The overall speaker may not sound so good. Most quality bookshelves focus on the midrange and tweeter and depend on a sub to pick up the lowest lows.