If it were me, I would either replace the speakers or get a subwoofer.
Take a look at the specifications of your BHK 250 stereo amplifier. At 8 Ohms (rating of your Tannoy’s), that amp is putting out 250 watts per channel, which is actually more than your speakers are rated. Tannoy recommends 25-200 watts for that speaker model. The problem is not power and a pre-amp won’t make any significant difference as you are already driving those speakers with 50 watts more than their maximum rating. Nor will the density the preamp provide make up for what I consider the shortcoming in your system…the speakers.
Moving onto the XT 8F specification, I see a problem. Those speakers are only capable of extending down to 34 Hz. The BHK 250 amp that you have extends down to 10 Hz. A preamplifier will make no difference.
You have a couple options and each is accompanied by some pros and cons.
- Replace the Tannoy’s with a Make/Model of speaker that extends much lower than 34 Hz.
- Try to integrate a subwoofer to provide a frequency response below 34 Hz
Personally, I have had very good speaker and amp combinations where I attempted to integrate one or more subwoofers. I have not owned your model of speaker ,so I cannot speak to how readily they’ll adapt to a subwoofer in the room. I have found this task uncannily difficult. I’m not trying to suggest it can’t be done as I am sure there are people here who are pleased with the subs in their systems, but for 2-channel stereo music listening where sound quality is of the utmost, I prefer a loudspeaker that is designed in such a way that the low frequency drivers are integrated. Time alignment has such an impact on speaker performance & sound quality. It’s exponentially more difficult to achieve with disparate systems.