First, make sure your LD is set on high gain. Stock, LD is usually set to low gain, you can easily switch to high gain moving the two gain switches on the bottom of the board (unscrew the 2 bottom screws on the front, same on the back, and the bottom cover will come out, easy). You have to move both switches down to make it high gain, don't move just one. Feel free to ask for help if you can't figure out how to do so, but it's actually quite easy.
Second, I'm pretty sure the LD needs more power than your iMac can provide, so I strongly suggest you to use a dedicated DAC or a DAC/amp combo to feed the LD. If you can't afford it right now and nobody around can lend you some, at least make sure you're maxing the iMac output volume in order to minimize the gain needed from the amp.
Third, stock tubes on the LD are 100% crap, and you can find cheap NOS matched pairs on eBay for under $20. Again, feel free to ask for some help.
If you're sure you've set the LD on high gain, your tubes are ok and and you're using a proper source with enough power to feed your LD, you should try to figure out if there's something wrong with your LD and/or your HE4XX. Try using the LD to drive other headphones like your HD6XX or HD58X, and compare to the sound you get from the DarkVoice, difference shouldn't be "dramatic": DV should sound a bit fuller, low end should be more present, and soundstage maybe it's wider, but not like night & day differences. Also, try finding some other amps capable of driving the HE4XX and test if there's any improvement in the sound.