Halfway in the room for seating may not be the best for acoustics, 1/3 or 2/3 positions generally better. Two subs is better IMO, and if of reasonable quality can be quite fine at your budget, too,, altho the price has climbed a bit in recent years. The HSU TN1 looks great so you could try one and save up for another if necessary....but I like the two RSL suggestion more as far as keeping in budget now. If your pre-pro/avr only has capability to set level/delay for one sub, a splitter will work fine. You don't need but one rca to each sub generally, tho
ps was thinking you had an avr, your integrated amp doesn't even have a sub pre-out, just regular ones so splitting the pre-out signals would require two rcas to each sub (assuming you want the sub to sum the signal to mono).