Sorry to have to tell you that you cannot expect any reply, help, or even an acknowledgement from any one at the Company regarding this issue, especially on this Forum.
It is a problem that has been spreading with the SlingBox PRO and PRO-HD for almost 2 years. It is a firmware matter as you surmised, but despite being fundemental to the advertised properties of the boxes, or perhaps because it is, SlingMedia will not discuss it here.
You will see that they reply to many threads on the Forum about other matters, and must have monitored the threads regarding the Freeview Tuner, but have steadfastly ignored it.
In particular look at my last comment, and if you ever hope for the matter to be resolved consider the action I have suggested there.
AND the earlier posts at
Someone from SLING has at last posted an acknowledgement.
and on a few other threads.
Pleased to get some acknowledgement from the Company, after two years.
I wonder if anyone believes it is a total coincidence that this momentous admission comes after the campaign has begun to bring this matter to much wider attention especially to potential new customers?
If, like me, you can see that there is a link, then PLEASE help me keep the pressure up. Having an acknowledgement after all this time is one thing, getting the Company to bother to get the HD and HD-PRO boxes fixed for existing customers is another matter.