Upgrade from ADS L1230

Hello, I am the lucky owner of a pair of ADS L1230.

I like the amplitude of the sound-stage and the tone is very balance with a sweet, soft and well defined mid-high.
What I am missing is definition on the low-end. I feel like the bass is not as punchy as I would like and in general I perceive some sort of sluggishness at the lowest frequencies. 

I would like to upgrade to a pair of speakers that would give me the same sound-stage and pleasant mid-high and at the same time better performance on the low-end. I was thinking about going for a used couple of speakers, having a 1500$ budget.

I am planning to driving the speakers with my actual amplifier which is a Threshold Stasis 3 with factory upgraded balanced inputs - recently fully serviced and restored (only psu caps have been updated, all the other components were within specs.)
My room is 12x18 (ceiling is 10' tall), with hardwood floor mostly covered by rugs.

Any suggestion?

You should check out a used pair of Vandersteen 2s. There is a nice pair of 2ce signatures listed here for $1100.00. I think they would be better than those 40 year old speakers that you are using now! Another plus is Vandersteen is still in business!
Those speakers tend to have a very solid/tight bass if placed/positioned properly in the room. Experiment with different distances behind and to the sides. If they are directly on the rug, use longer spikes and anchor them to the floor through the carpet. If they are on the hardwood, use spikes but put something between the spike and the floor that decouples them from the floor. Many here use Herbies products with decent results. You should not need a subwoofer with those speakers unless you want to use them for home theatre and movie sound effects.
My experience with ADS speakers is similar to @kalali.  They are known for really good bass, so something is amiss here.  I wonder if this could be related to the age of the speaker or room problems.