While what people say about HiFi systems is correct, they are being audio snobs in my opinion.
If you start by treating a HiFi as FRFR - with High and low cuts typical of FRFR - i.e. cut below 100Hz at 24dB - in other words a drastic cut, and the same above 5.5KHz - use the Global EQ - (yes, there are 100 arguments why you should do it per patch - and they just confuse the inexperienced) then it will start to sound better. If your HiFi has a typical hi and low boost, you might end up at 120Hz and 4.5KHz - but you will be in the ballpark. You probably need a mid boost too - just a few DB over a gentle curve - you will need to experiment - but maybe at 1K - and you should be able to get a reasonable serviceable sound out of your Helix.
Anyone who says just plug it into your guitar amp is so far off course it's not funny - all the presets have amp sims - and are not designed to be plugged into a guitar amp.
The HX Effects is designed to work with a guitar amp and that is why it does not have amp sims.
Yes, you can use a Helix like HX Effects, but you'd be crazy now the HX effects exists. The Helix simulates guitar amps and speakers - which needs FRFR - full range flat response sound system. Yor HiFi is probably not true FRFR, but it's closer to FRFR than a guitar amp!
What this person is complaining about I'd say is the same thing - call it Fizz or harsh - that people who don't set up correct EQ always complain about.
I use FRFR for live, but I plug into my stereo at home and can get quiet a decent low volume sound - studio monitors will be better sure, but a good quality HiFi is pretty close to studio monitors - all things considered - you just got to realise the emphasis on extra thump and top.