Manu, I hate to say this but this a perpetual problem with all rear-ported speakers in small rooms with not enough space for the bass waves to propagate and subsequently dissipate.
Correct me if I am mistaken, but Isnt the polk rti a5 front firing?
Usually backfiring have an even more significant issues with distance to back wall.
If you are using an AVR, try doing bass settings.
It helps a lot without compromising the bass and mid range. Set the speakers to small (very important) and change the crossover to 80 hz. Trust it helps a lot.
sorry, but setting speakers to small usually in most receivers and amps, it internally overrides the crossover. When set to small it defaults to around 80, and tweaking it is no use in most receivers. If you want to set crossover then
set it to large!!!
If you have a subwoofer and crossed it at 80 Hz or even may be 100 Hz can help.
Yes.
But only to some extent. It is easier to get good tight bass from good sub placement than through an FS. One of the not so well discussed horrors of getting an FS. Placement of FS in some rooms becomes extremely crucial, especially for bass.
Especially if the right side of room is causing the issue.
It is not just a problem of right or left. It is the whole room.
I don't think the spikes will help, unless the speaker is not exactly flat to floor and causing vibrations. Also, I think spikes work best with carpet floor to give better speaker grip. I haven't used spikes so can't comment for sure.
A nice thick plank of wood under the sub & FS would help to some extent. MDF is great, with supports under the board.
If you
can get this, then it wud be great. But nuthing great about it that DIY cannt achieve.
For bass traps, it can come cheaper if you are looking into DIY. Although you will need very thick, 6"+ to absorb the low frequencies. There are tons of info on diy bass traps.
Thick rigid fibreglass, 2 panels of 6" each or one 12" panel would do great. But you would require to do this on every damn corner, and sides.
Again, without measurements dont expriment with your money
One which could help a lot if you room is not too big, try these in order.
1. Bring the speakers front. Experiment, 2 - 6 ft from back wall. Even moving 6 inches in any direction can make big difference. Watch the side walls, the ratio of distance from side walls to back walls is crucial. (But dont be rigid with anything. Experiment, experiment & experiment)
2. Crossover bass away from the main speakers to the sub, experiment with different crossover frequencies. Do not play all the bass from the FS. This is mainly because it is easier to place a sub. So before you do that, you need to place the sub right!!
3. Treat the wall behind you. This can help a lot, more so in a smaller room.
4. Bass traps.
(Before you do 3 & 4 you need measurements to understand what is the problem area before you spend your hard earned money)
Trying to find a link which I bumped across recently, it can help a lot. Will post it If i do find it.