Why not use a good y adaptor like from Cardas. I use one in my system to feed a sub and it sounds excellent
If the sub does not provide speaker level inputs, or if the good suggestion by Mesch is not feasible for any other reason, let us know the make and model number of the preamp, the sub, and the two amps. Hopefully we will then be able to determine the output impedance of the preamp, and the input impedances of the sub and the two amps.
Also, let us know the approximate length of the cables between the preamp output and each of the other components.
I'm not sure what is meant by "the pre-amp does not get enough of a signal to be heard," but assuming that the problem is poor sound quality coupled with a slight to moderate reduction in volume the cause is likely to be that the combined impedance of the sub and the amps or equalizer is too low to be driven properly by the probably high output impedance of the tube preamp.
The line-level input impedance of powered subs is often very low, in the 5K to 15K area in many cases. In combination with the input impedances of the amps or the equalizer the overall load impedance seen by the preamp will be considerably lower than that, which would be too low for many tube preamps to drive with good results.
If so, and if the sub does not provide speaker level inputs, you would have to either replace the preamp, or insert an active buffer stage providing low output impedance between the preamp and the other components. A high quality active buffer stage is likely to cost a few hundred dollars or more. The least expensive buffer stage I am aware of is the Yaqin SD-CD3, which various sellers offer in the area of $175 to $200 or so. I have no idea as to its quality, though.
Audioquest makes Y adaptors in various male/female configurations. One thought I had was that I am not sure of the impedance issues of driving a single pre output into two amplifiers however. Given the high input impedances of amps probably not a concern. I do know that Al can provide a more than adequate answer.