Symptoms

1.) Initially we were not able to get smartobject services up and running (400 Bad Request)
2.) After fixing that, we struggled a little to get filtering working on the smartobject services.
 

Diagnoses

1.) The initial issue was because the SPNs were not set on the right account.
2.) After getting the services up and running, we struggled to get filtering working while using a list method with the smo services.
 

Resolution

1.) After setting the SPN on the K2Service account instead of the K2Workspace account, we were able to access the smartobject services.

2.) After struggling with the filtering for a while, I went back to basics. I applied the filter in the smartobject services tester tool. I copied the resulting XML, pasted it in notepad, removed the return characters and twiddled with that a little.

In the end, the following example filter worked as expected...
http://hURLGoesHere].com:8888/SmartObjectServices/rest/DBNAme2 on machinename/Tables/Tdbo]./TableName]/List?filterXml=<filterexp><equals><left><propertyexp name="FirstName" sotype="Text"/></left><right><valueexp sotype="text">Bob</valueexp></right></equals></filterexp>