Audio Technica ART9 sounds awful

In addition to the tubby bass and the shifted soundstage, is imaging vague and diffuse?  If so, it would be suggestive that + and - may be reversed on one channel, either within the cartridge or in your connections to it. 

A means of checking for that possibility that may be more convenient than changing cartridge connections would be to swap + and - at one end of one speaker cable.

Also, I'm wondering about anti-skating, which might be more critical than with your previous cartridge as a result of the ART9's higher compliance.  When you view the cartridge from the front while it is in the groove of a rotating record does the cantilever appear to be deflected significantly to the left or the right, relative to the nominally straight ahead position it should have when the stylus is lifted off of the record?  If so, you may want to readjust anti-skating per the procedure described in my two posts dated 4-11-2016 in this thread.

Regards,
-- Al
 

What tools/process did you use to align the phono cartridge?

My money is on Almarg's hypothesis that the two channels are 180 degrees out of phase with each other.  Even if you cannot prove that by visual inspection, you might try switching leads on one channel just for the heck of it.

I am no authority on the ART9, but I found on the AT website the following: Compliance = 18 @ 100Hz; Static compliance = 35.  Wally Malewicz's web page says that the compliance at 10 Hz (which is the value to use in the resonant frequency equation) is related to the compliance at 100Hz by a factor of 1.5-2.  So one would predict a compliance of 27 to 36 at 10Hz, not out of the range of the above quoted number.  However, Wally also says that the static compliance is related to the compliance at 10Hz by a factor of 0.5, which would lead us to think that the compliance at 10Hz is about 17-18.
This topic is quite messy.  As a person who likes to boil things down, I note that the resonant frequency is inversely proportional to the square root of the product of M X C, which actually leads to the finding that there is quite a lot of tolerance in the relationships that allow one to end up with a tolerable resonant frequency. (Try plugging in a range of real world values for M and C and take the square root of the product, to see what I mean.)

+1 on almarg's idea. I don't think break-in is the issue here. the cart should sound great from the get-go, and get better.  

In my post above about the plasticity of the relationship between effective mass and compliance, I might better have written that one should make the calculation of Fr for a variety of values of M and C, in order to see that you can get away with some fairly unlikely combinations and still be within an "acceptable" range.  In part, this depends also upon what one considers acceptable.

Anyway, I still think Almarg nailed this particular problem. Which has nothing to do with M and C.