Just to note that a headphone amp can't drive speakers. Only a speaker amp can do that.
But if you mean you want the capability to hook up active pro audio speakers, that is a good reason to go with a balanced setup. Some pro audio speakers (e.g. JBL LSR series) are prone to picking up PC noise when used with unbalanced connections. But with other speaker setups, unlikely to help.
As for the notion of being able to drive almost any headphones, the headphone community always moves to the latest, most powerful amp and hypes it that way, as well as promoting the general sentiment that balanced is the only solution for driving difficult headphones.
Single-ended amps such as the Schiit Heresy come very close in headphone amp performance to the 789. And even the Atom has sufficient power to drive 99% of headphones.
So personally, I would not focus on "balanced" since you are looking at the used market. There are many amps, some of which are not balanced, that can meet your needs for driving future headphones. I would look at everything about an amp.
Yes, sorry I meant as preamp.
I understand "balanced" can be a marketing buzzword and many products implement it badly just to slap "balanced" on the box. I generally follow value, living in a country where minimum wage is the price of a new 789 I have some immunity against hype, even if I can afford it, it makes me feel bad
For the ~$100 family of amps I wouldn't get them 2nd hand, and to anyone outside the US they're bad deals because:
- Not a significant drop from MSRP, usually go for $80-90
- Currently JDS and Schiit are only shipping express which is expensive
- Require additional 220V plug because you will only find those offers for 110V, so another shipping bill unless you found a kind seller who would do you the favor of consolidating the packages
So not worth it at all 2nd hand, though $250-300 amps like SMSL SP200 and Drop 789 go for $160-220 and take 240V, that's a nice value IMO.