IMHO they owe you a working TF10, or better. Not a lesser product.
Ask their name and use it throughout the conversion. Be polite, but firm. Always keep your cool.
Don't be overly focused on the 900. Keep telling them you would be fine with a new or repaired TF10, but the 600 is unacceptable.
Ask them to put themselves in your shoes.
Read them the terms right off of the warranty card.
Ask them to explain to you where the purchase price comes into these warranty terms.
Ask to speak with someone with the the authority to honor the warranty as written.
Write a letter to the head of the UE division (or the President of Logitech) asking them to please solve the problem. Send it FedEx (2-3 day service is OK).
If all else fails tell them you will be taking them to small claims court, and do so if needed. They'll likely take care of you as soon as their legal deptartment gets the documents from the court.