You have two other offers pending, and this offer is low? I'm not quite sure from your comments whether you already have better informal offers, which you fully expect to be made formal, or whether the other institutions haven't named figures at all until their offers are formal, but you expect them to be better. I'll assume the latter, but in the former case it's even easier to be tough with these guys.
From a negotiation POV you should probably say, "I will not accept this offer at its current level, but I am open to further negotiation if you're prepared to make a higher offer on salary and startup". By all means wrap that up in some stuff to remind them how much you like and respect them--there's no need to be blunt just because your position is tough--but state your position. This satisfies their request for an informal response to their informal offer, and they can decide what to do next.
You could add that the reason you won't accept this offer is that you expect a better offer elsewhere, because that will help convince them that you are not bluffing. The trouble with negotiation is that they can only act on what they believe, not things that you expect or know to be true but they're unsure of.
Alternatively, you could say, "I cannot give you a response until I receive the details of two offers pending, which I expect to happen around such-and-such date". This doesn't satisfy their request, but it does leave them with a clear choice to make: they either cut negotiations now or they accommodate you. If they cut negotiations, then most likely you were never going to reach a deal anyway.
Finally, you could say, "I can informally accept your offer pending your detailed formal offer". Then if you get a better offer, call them back and say, "I have received a better offer, so I am now rejecting yours". This is kind of a jerk move, and they'll dislike you for it, but they'll also learn that this is what happens if they rush people into informal lowball agreements.
I understand that you don't want to be rushed, but if they're pressing for an answer and they're not moving on their offer, you have to consider the possibility that they just don't have enough money to fund the position in competition with these other institutions making offers at the going rate. Every negotiation contains the possibility of not reaching agreement. In which case, the sooner they move on looking for someone willing to do the job for cheap, the better for them and for you.
I suppose another reason they might be rushing you, is that they think you're probably too good for them and kind of expect you to turn them down, so they don't want to lose their second-choice candidate. It's really none of your concern who their other candidates are, but it's legitimate for one of their goals in the negotiation to be, "a quick answer". But then in their own interests they should do everything they can to help you decide quickly, including expediting a formal offer and negotiating salary. If they want you to be in a hurry but don't seem to be in a hurry themselves, then it's more likely this is just a tactic on their part to weaken your position by having you negotiate without the other offers in hand. If all they want is for you to say, "I won't ask for more salary", before you've seen your other options, then yes they're being aggressive.