Which equipment did you swap? Which vendor are you using? Are your system parameters the same before and after the swap?
If your statement about the MCS is true, it means it’s probably a parameter setting. Are you sure you’re not using a static MCS algorithm after the swap?
Also, if the MCS is always MCS6, then that’s the reason for the low throughput, ‘cos the theoretical max. data rate for MCS6 is around 30kbps per TS. In addition to this, you will have more retransmissions in poor C/I areas.
Let me know the answers to the above questions, then I’ll see how to help.