DXM is an inhibitor, and taking it with codeine is blocking some of codeine's conversion to it's active metabolites.
DXM causes it's own euphoria in it's own right, which is why you felt great.
If you take the DXM with an opiate that is already orally active without conversion, you will see a big difference.
Anti-histamines only stop nausea, and add numbness and drowsiness to the opiate, not euphoria.
And no anti-histamines should be added before or too early after the codeine, or you could reduce the amount converted in your liver.
Alcohol and other inducers greatly add to the sedation and numbness of the opiate, but it boost the amount converted, rather than inhibit it. When it comes to opiates with more powerful metabolites, using inducers GREATLY add to the euphoria. And there are better inducers out there than just alcohol alone.
Soma, and other drugs that induce similar actions to barbiturates, increase this conversion so much, that codeine can feel like hydrocodone and other more powerful drugs that act on the same receptors.
Many have concerns on duration after inducers are added. This is of no real concern, because there is so little difference.