You should also be wary of CYP2D6 inhibitors.
CYP2D6 is the liver enzyme that converts Tramadol to O-Desmethyl-Tramadol, the latter being its active metabolite which acts as a stronger opioid but has less of an effect on the serotonin transporter. Several antidepressants (like Fluoxetine or Duloxetine) act as inhibitors of this enzyme, so taking these with Tramadol exponentially increases the risk of a seizure since you've already got a higher baseline serotonin level from your antidepressant, *and* you're potentiating Tramadol's own SNRI activity.
I would also wager that the risk very much depends on how your doses are distributed throughout the day. Taking 6x50 mg IR at regular intervals (thus giving your liver time to metabolize the Tramadol) should be far less risky than taking 300mg IR all at once.