How do I reduce the overall gain in my preamp ?

If it is adjustable than you can reduce it.

You can also reduce the gain with varing anode DC feed with potentiometer or in SS case where there are few possibilities of implementing the amplification stages but in any case you should use the output DC feed. In the most common example with emitter-coupled stage you should apply the variable DC voltage to Kollector.

In both of the above cases the large variations of the gain are not acceptable since your offset is set to be for a particular gain i.e. you should vary the offset as well accordingly to the DC applied to the output electrode. That's why the variable gain is usually implemented stepped with for example 3...4 settings where you vary the output DC feed along with the offset.

This is tedious unless you really know what you are doing. I would not attempt if you are not familiar with electronics. The circuit is "tuned" for all the parameters involved - you change one, you change everything. Just turn down the volume or get a different preamp IMO.

I put a passive line stage in my tape loop - for casual listening i have remote control also. works fine. When i'm feeling picky about sound i simply flip the switch turning off the tape loop. If you can accept some minimal signal loss (there will always be some) you can put a passive preamp of high quality between your amp and preamp but you must deal with the cable between the passive and the amp carefully to minimize the loss. I've done this with some success - not as funky as it sounds, but you'll get booted out of the audiophile corp if you tell anyone you did it!