And then the old hand-me-down amp/receiver breaks down and here you are with highly unwanted (new or second-hand) passive speakers. Now you have to see you can rid of the passive speakers (and take a loss) or buy a new amp instead of active speakers.
Just the fact that someone gives you some old equipment for free because they don’t need it any more, does not mean it’s the most clever move to add new material to it to use it. If they gave the amp to you WITH some passive (monitor useful) speakers, that would be a different situation. You could use that until a part failed and trash it all or until you had money to move up to active speakers.
FYI: The Denon was launched in 2003 I think so could be up to 13 years old. Not a problem in and of itself, but the idea that you can actually sell it next year is crazy. Well someone might pay 10 bucks to come pick it up for some dark reason, but no money there that will aid you in buying your new gear.
In short. If you buy passive speakers to augment the free amp, you should consider that money gone and just hope the combo lasts long enough to save up for an active set. Alternatively you could try to source some passive speakers for free from others in your network and use this completely free solution until you are ready to buy what you really want. Intermediate solution could be to source a cheap pair of used active PA speakers from a good brand (10″ for example) that will let you practice with PA sound (monitors are really not all that for lengthy practice session imho, they are better suited for production or detailed rework and such) and you could use them to play at house parties and such as well with them.
.
