Atma-Sphere amps reproduce low end strings as whole and correct as I've heard.
Power cords make a nice difference with Atma gear, but not as much difference as I've heard them make on other amps. I use Shunyata Anaconda CX cords on my MA-1 monoblocks.
Try a couple pair of Ken-Rad VT231 6SN7s; they help with mid- and lower bass tonality and articulation.
Also, depending on your speakers, a set of Speltz autoformers can help with that last bit of weight and tonal completeness.