In this work we present both experimental and theoretical investigation of gas gains measured in single anode wire cylindrical counters filled with Xe + 5% TMA (trimethylamine) for the mixture pressures between 250 hPa and 1800 hPa. We observed stable operation at all pressures up to the maximum gas gain values as high as 10(3) (10(4)) at 250 (1800) hPa. The experimental gas gain data have been fitted with the output of Magboltz software to investigate additional ionizations, developing in avalanche formations, due to Penning energy transfers and secondary processes (feedback). Calculations manifest that the choice of TMA as a quench gas have a significant effect on the reachable gas gain. The obtained results are expected to be favourable for micromegas-TPC operating in Xe-TMA mixture.