The best speakers I've found for deep bass is the 4 ohm line of audiophile speakers from Tang Band. They are available from Parts Express. Because they are 4 ohms, check with your decoder manufacturer to see if it is compatible. I use Loksound decoders, which are rated for 4 or 8 ohm speakers.
Look into the TB modules instead of loose drivers. The modules come with a sealed enclosure and a passive radiator that greatly extends the bass response.
I use the T1-1925S module in brass diesels such as GPs and Alcos. The smaller T0-2008S fits in Atlas SWs, but it is easier to overdrive. In P&D single tower F-units I will be milling a hole through the frame for the T1-1942SB.
In a steam tender you may be able to fit a larger T1-1931S or even a new T1-2025SC.
The PDF data sheets all have dimensional drawings. By the way, most modules have mounting tabs on each end that can be sawn off without damage.
Link to iPhone video of TB 1942SB and Loksound 567B/C file
Link to iPhone video of RS36 and TB 1925S
Link to iPhone recording of an Atlas SW9 with TB 2008S speaker
By the way, if you go with Loksound and run diesels in consists, by varying the playback speed of the prime mover and dynamic brake by 1 or 2 percent in each unit, the consist won't sound like one decoder driving multiple speakers. That's because the #1 cylinders won't be firing at the same time.
