While transistors do clip more harshly than tubes, under normal operation you shouldn’t be driving the stereo hard enough to cause signal distortion. That argument basically boils down to “if you crank the stereo up so loud it sounds like crap then tubes sound less crappy”. It’s not a valid comparison for normal listening levels.
The sound you get depends more on the design of the amplifier than what technology you use, though it is easier to design a less distorting amplifier using cheap parts with transistors than it is with tubes. The “tubes are better” snobs are usually talking about high end amplifiers where the signal distortion is so small either way that the tube vs. transistor argument is rather pointless. If you want a cheap stereo though, transistors tend to sound better. If you want a cheap guitar amplifier, tubes sound a LOT better.
As for the OPs stereo, if it’s a high end stereo from the 70s and the capacitors are still good (as has already been mentioned) then it probably will sound as good or possibly even better than a cheap modern stereo. Speaker technology has improved quite a bit since the 70s though. Tiny bookshelf speakers today sound significantly better than tiny bookshelf speakers from the 70s, and big high end speakers today sound better than big high end speakers from the 70s. Tiny bookshelf speakers of today do not however sound better than big high end speakers from the 70s, generally speaking. You may want to replace the speakers (or recover them, which I see on preview someone has mentioned while I’ve been typing and distracted).
A 70s stereo system will probably be less efficient, turning more electricity into waste heat instead of sound. The difference probably won’t be enough to be noticeable on your monthly electric bill though.
What is he playing through this vintage stereo system? Vinyl destroys more and more of the recording every time you play it, has rumble, wow, flutter, and occasionally static pops, and also has a poor overall dynamic range compared to a CD (a CD also does not suffer at all from rumble, wow, flutter, or pops). The only thing vinyl does better is very high frequencies, which most people can’t hear. If you are one of those people who can easily hear those annoying mosquito ring tones on cell phones and you have a thing for piccolos, then maybe you’ve got an argument for vinyl. Otherwise, go digital. MP3s though distort the sound, and depending on the sample rate, can be worse than vinyl. Any tape format is horrible, suffering from poor dynamic range and that ever present tape hiss. Tape is probably on par with the worst MP3s, and even then MP3s, while distorting the sound, won’t have that annoying hiss.