Experimenting with Speaker Room Placement

My room is similar to yours, and I did the same thing initially.
I ended up placing my speakers at the middle of the room-using the shorter wall. The sound is very good, with almost no bass nodes.
It also looks good, as this is the division between living room and dining room.
B

FWIW, your bass problems are likely a problem with placement of the speakers in a bass node. Or your listening chair is placed in a bass node, or both.

I have a 13x19 ft room, have the speakers on the short wall out about 66 inches and about 96 inches apart. My chair is about 48 inches from the back wall. If I move either my chair or speakers back towards the wall behind them I get a bass increase that can be very noticible. I also have noticed that toe in as well as placement affects the potentially  'fatiguing'  issues you refer to. I have side walls well damped with bookcases, drapes etc to kill sidewall first reflections. IMHO getting those speakers well out from that back wall is a major issue. 

I also tried the long wall. I found the sound rather sterile. I think it killed the natural room acoustics.

I found this with several sets of speakers, none of which were yours.

Nearfield or not, short wall or not can largely be personal preference, but it can also can be somewhat dictated by your equipment -- most of all speakers.  For instance, tower speakers often require a greater listening distance so their drivers, which can be spaced farther apart, have enough space to come together -- not as much of an issue with something like 2-way monitors. 

Personally, I'd put the speakers along the short wall if possible as you can listen in the nearfield or not, and you'll have more freedom to pull your speakers out further into the room (also need to give them some space from the side walls).  This not only helps improve soundstage depth but can also help mitigate bass overload, which may be what you're experiencing.  If you haven't already done so, try pulling your speakers -- especially the towers -- 3, 4, or even 5 feet out from the wall and you should notice a significant improvement in tonal balance along with possibly a greater sense of a 3D soundstage.  It may also help your speakers disappear more as a sound source as you get them further away from reflective boundaries.  Before I had a dedicated listening room I used to pull my speakers out for serious listening and put them back to avoid divorce.  Some things are just worth the effort. 

Other things you can try are toeing the speakers in more toward the listening position or plugging the port if the speakers have them (although this doesn't work well with many speakers).  If you're on a suspended wood floor you can also try getting some marble or concrete slabs (and/or audio oriented footers although these can get more expensive than slabs) under your speakers to help decouple them from the floor.  Also, don't place the listening seat directly up against a wall as the reflected sound can muddy things up considerably -- even a few inches can make a big difference. 

I'd think with your size room this should be a manageable situation to a good degree even without room treatments if you have placement flexibility.  That said, I don't know of a room that wouldn't benefit significantly from the use of some well-placed treatments (assuming no electronic room/speaker correction is being used), so that should eventually be a part of your plan regardless.

Just some thoughts off the top of my head but hope some of it helps, and best of luck.