To determine if the loss in gain seen by the new eom driver (SN:06) was due to the driver itself or just because of the addition of the bias tee, I took the transfer function of the old eom driver with and without the bias tee. This plot shows the LTspice simulated transfer function with and without the bias tee, as well as the effect of adding the bias tee to either driver. Here are some take-aways:

• In both cases, there is a similar reduction in the peak height and increase width of the peaks.
• This indicates the SN06 board is not the issue and it is in fact the bias tee causing this change in gain.

It seems we cannot introduce a bias tee without facing a reduction in the gain of the driver.