The reason I think it's comb filtering (not that I know that much about it) is that in my old room the monitors were placed closer to the front side of my desk and there was not the bass loss I'm experiencing now when they are at the back side of the desk.
What do you mean by "
the true response of your monitors and since you have a lot of HF absorption stepping away from the sweet spot of the monitors could could exaggerate the lows."?It is indeed hardwood, I have a rug in the middle of the room
(and a bed in the back of the room).I was thinking that aswell. Is it "safe" to place them vertically? According to the user guide it says "
Place the BX8as vertically with the woofer on the bottom. Placing the BX8as horizontally is not recommended." I have tried that since I moved here, it was alot of "phasey hollow fast feedback delay" at first but now it's not nearly as much - I don't know how much is too much tho

Thanks for your help!
I was saying that it may not be bass loss, considering that the old room especially if it had no acoustical treatment could have had a bass buildup (there were standing waves in the lower frequencies that were in phase and actually exaggerated the lows), its pretty rare but could happen. As soon as you added traps, especially the bass traps, the sound is being absorbed instead of reflected which gives you more of the real monitor sound an not reflections from the room. If your monitors are at least a few inches up off the desk, having them back won't do anything, what you may or may not have noticed is the human ears natural tendency of the proximity effect.
What I meant by your quote is that since you have acoustic treatment in your room, you are going to have a lot of HF absorption, as HFs are very directional and LFs are not, the highs will be absorbed while the bass since they tend to fill a room more will be more apparent or exaggerated.
Placing your monitors vertically will not physically hurt your monitors. The reason they say that is because most new monitors have a wave guide that helps disperse the HFs (although they really don't need any help in a near field situation). The BX series has a universal direction dispersion pattern, which means they disperse the same all they way around no matter how they are oriented. When you have the monitors vertical they disperse the sound at different location compared to how elevated you are compared to each crossover, when they are horizontal, no matter your elevation, the crossovers will always be the same distance to each other (that an the shape of the human ear is made to hear different sounds on a horizontal level, which means that if you were to move to the left and right to simulate the effect of having them vertical wouldn't make a difference in sound as your ears pickup sound at a narrow horizontal level, that's why you hear the sound differently if they are vertical because the other crossovers are not the in that narrow field. Setting the speakers up horizontally sets up all the crossovers in that same field. Also if you notice the majority of producers, audio engineers have their monitors setup horizontally on their consoles, usually because the speakers would sit to high but to also disperse the same frequencies over the same narrow horizontal band in which we hear.
Some of the phasiness is ok, your not going to get rid of it even with acoustic treatment if you have many parallel walls and 90 degree surfaces, a little is fine, your best bet is that if you sit in your attended spot of mixing, clap your hands and the phasiness sound more light and ambient (not directly in front of you) its fine, as that will only cause bass cancellation if you don't have bass traps, get the bass traps behind you (the direction the monitors are pointing the sound) and in the corners of the room. If you can walk around and clap, find where the echo/cancellation is coming from and add HF treatment, it will sound like you are in between two solid concrete wall and clapped really loud, we all know what that sounds like. The area where that gets the loudest and where you feel that you are caught in the middle of the bouncing sound, place your treatment on those two adjacent walls that the bouncing sound is coming from. Its usually the two walls that are parallel to the monitors (the walls to your left and right) that cause the phase issue, but HF treatment needs to be done a little to the front and back of your mixing position.