Any tips on designing an audio preamplifier for low frequencies?
1 - Quite probably

2 - This search says its quite possible also.

3 - While C1 and C2 block DC they will also be a RC filter, which will hinder low frequency reproduction.

You'd need higher values capacitance to reproduce lower frequencies. This calculator can be quite useful to determine such values.

Can you try to build the amp first and try it on a waveform generator before trying it on your phone? Have you tried measuring your phones headphone output with no load?

This way you'll isolate the problem and make sure each part of your chain can do wat you want before trying to make it all work together.

p.s.: Be careful when building an amplifier which reproduces almost DC, most speakers/headphones cant handle DC for long.