Thank you for your question.
The point regarding the improper service of summons has likely be waived by the fact that you have appeared at a hearing before the court already. Regardless, if you want to attempt to quash the summons for insufficient service, then you would need to file a Motion to Dismiss
for Lack of Personal Jurisdiction based on the improper summons.
The only reason you would do this, however, is if the statute of limitations
has passed on the debt claim. Illinois, the statute of limitations on a written contract is 10 years, unless this was for a credit card, and then the statute of limitations is for 5 years.
Do you know what the debt they are trying to collect here is? If so, when was the last time you made a payment on the debt? If it is over 5 years ago, then you would want to file the Motion to Dismiss for Lack of Personal Jurisdiction and state that you are making a Special Appearance to argue that you were never properly served and thus the case should be dismissed with prejudice because the statute of limitations has passed.
If the statute of limitations has not yet passed, then fighting the improper service of citation/summons would likely be futile as they will only serve you again.
Thus, in that case, the proper thing to do would be to file your answer and assert your defenses, as you had previously stated.
You said that the agency said they do not have to prove the written assignment of the debt. That to me sound absolutely ludicrous. Whether or not the Illinois Collection Agency Act applies to them, since you are not in contractual privity with them and they are suing you based o a contract, they have to establish the valid assignment of the contract or they cannot establish that you owe them any money. So, their argument is non-sequitar. Did the judge issue a ruling on this argument or did the judge simply state that he will hear the argument at the trial?