Is this an alternative to Stromtank and PS Audio ?

Any inverter that advertises a true sine wave output is a good starting point. The others have enormous amounts of harmonics because that's less expensive to build and not a problem for electrical (NOT electronic) tools and appliances. Additional downstream low-pass filtering is always a good idea. 

Anything with enough capacity to run a power amplifier will require an automotive capacity battery to provide 120V X 15 A - AKA 1800W or 1 standard AC circuit. A 20A 120V circuit needs to supply 2400W. That's equal to 150-200 Amp/Hours at 12V per hour of operation times the efficiency of the inverter. So, an 800 Amp/Hour deep discharge marine battery might run a good-sized system for an evening before needing a recharge.

Worth it? your call, not mine.

It's very simple.....it is the watt hours of battery/batteries you have available divided by the load.  If you system draws a constant 500 watts......then 5000 watt hours of battery (two 200 amp hour batteries....ie 400 amps x 12.6 volts) will let you run at least 8 straight hours without recharging.  Most do not listen that long.  You can also charge while listening but it does not sound as good as disconnecting the charger from the battery.  If your system only draws 150 watts then you do not need much battery.

The only way to know if an inverter system sounds best is to try one.....the info on my site shows a setup that is mindblowing good.

Using batteries to power an inverter offers 0, nada, none, zip advantages to drawing from household current. Both will ignore spikes, lower voltage, higher voltage, and frequency variations. Just with the battery “powered” version, you get to pay a lot extra.