USB 1.0 = 1.5Mb-12Mbps That bits NOT Bytes. So 1.6MB/s is about right.
USB2.0 I've seen up to 30MB/s when it's max theoretical is closer to 60Mb/s. It'll never get much higher than 30MB/s in real world.
EIther way, you need to JUNK that stuff NOW. Get something that is at least USB 2.0, eSATA, Firewire 800... Although those all have no place for a server... You should be using SAS, or SCSI for reliability sakes.
A nice little boost I've found for incrementals is to NOT use the archive bit, but rather "Modified time," and the NTFS Change Journal. But I suspect there is something like your AV settings are not set right for BackupExec, or an incorrectly configured RAID5 array (diskpart align= command, small sector/cluster sizes, no write-back cache, etc) Or even updating BackupExec, but not updating the agents afterwards...