Todd,
SSXM has a component called a SSXM Gateway. This component (service) can take the place of your 2004 SS. It provides liccensing to pre-XM products. Assuming SSXM is configured to allow license checkouts the Gateway can provide checke out licenses to pre-XM products. The Gateway records the checked out licenses and normal pre-XM usage and then reports it up to the main SSXM service. Please let me know if you need any further information and I will be happy to help.