Well Hi fi system to me means a great monaural system. with less than 1% total distortion from the beginning of the chain to the end. It must be able to reach concert level with 10 db of head room. Signal to noise should be good enough when playing a source so no noise should be heard at concert level. Back in the 50s thats all we had was mono Hifi and yet the speakers could not reach the 1% spec and you were lucky if they could reach the 10 % spec. 30, to 15,000 HZ was just about the limit of the science for a total system.
I have about 100 mono albums either as LP's or CD's and FM selections played once in a while in mono in which case I use the right speaker only. Remember two speakers playing the same identical signal reduces the intelligibility factor by two.
If you really want to know how good your stereo system is play a mono CD, tape, or LP. The image between your speakers should be very narrow and that image should extend from the very highest of frequencies to the upper bass around 250 HZ. If the center image is vague covering say over a 1/3 of the area between your speakers or larger then your speakers, room acoustics and listening position are very suspect. Some times even horn loaded speakers where the crossover frequencies are to low for the capability of the mid-horn or full HF horn the imaging will be vague and not as focused as it should be. With point source speakers you have to sit in the direct field of the speaker for sure to even begin to get the correct focusing.
The only horn system I have ever heard for a home system that meets those demands if the Klipsch Jubilee. Altec can't, DD JBL can't, EV never could, and though my favorite smaller horn system, the Cornwall, can't either. My HT Line arrays come fairly close, but the stereo line arrays definitely does at 15 or more feet listening distance.
I will admit that there are a lot of folks who don't want that highly focused critical sound presentation. Preferring a sound that surrounds them in an illusion. Bose 901 for instance does that and made Bose famous. But thats not Hi FI. Maybe its mid fi or called something else.
Stereo HiFI is something totally different to be discussed when some one asks about Stereo Hi FI.