Would increasing the gain in the amp give more weight to the sound? Any negatives?

Well, from what I can get there doesn't appear to be any components and cables mismatch, I think it comes down to the fact that passive preamp is no way to go if you want full-bodied sound, and this was confirmed by many here who tried it both ways. Most though not all prefer active preamp.
I don't think I will ask the designer to increase the amp's gain, it may throw the amp off the balance with little or no benefit, it would be a gamble, I guess. The resolution and soundstage decrease when running the phono thru the deck is slight, excellent Purist cables help a lot. I'll just wait till I am ready for a major upgrade.

I think it comes down to the fact that passive preamp is no way to go if you want full-bodied sound,

You countered that argument by your saying the cassette is fine. 
I would also hazard a guess that your PH1 will have a better stronger output stage and with more voltage than the old Naka's got.

Cheers George

I was talking about phono not deck. By the way, I didn't mention it because it doesn't bother me, but my CEC belt drive cd player also sounds better in this respect when routed thru the deck. It seems that the deck doesn't mind passive preamp, though I didn't compare its sound with an active one.

Try reversing the polarity of your speaker cables. You might discover that your system is reversing the polarity, and by flipping the speaker cables, you suddenly hear a richer sound. My system needed it. And, the one before that did too.

Genez has hit on a good point there, but it could be just one channel that's out of phase with the wiring in the cartridge/arm/or even the PH1. Just try reversing the polarity of one channel first. 

Cheers George