Is this an alternative to Stromtank and PS Audio ?

Possibly. You can set it to charge in the middle of the night, then run it off batteries when listening to your stereo. Don't be surprised if it has a loud fan though. Also don't be surprised if when someone turns it on/off for you, you cannot hear any difference. YMMV. It is a small investment overall and has lots of other uses.

One option is to have Amazon deliver an enormous amount of rechargeable 'D' cells with a OTR 42' trailer and set them up in the garage....

The neighbors Will talk amongst themselves.... ;) 

Yes, and mostly no.

All in ones are good sonically (every brand sounds different.....some are OK, some are good and some are great....just like everything) and some are probably equal to the Stromtanks or even better (however, most of them have noisy fans).....see review by Tom Lyle in the link I provide. However, my friend used the 3000 watt Yeti and that was beat by the Ecoflow pro....which was then stomped to death by the Giandel inverter plus LifePo4 batteries shown. He also uses a Puritan line filter after the inverter for even better results.....all inverters still put out some noise.....the Puritan takes it way, way down. This is revolutionary.....you will not believe the transformation of your system doing what my friend did. You can experiment for almost free....the inverters, batteries and Puritan are all returnable.......do you want to blow you mind?

http://tweakaudio.com/EVS-2/Inverter_Power.html

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.