How to supply a source to a headphone amplifier for the best sound.

I bought my very first headphone amplifier recently, the Massdrop THX 789 and had problems getting the music to get loud enough to satisfy me. My first source was direct from my cd player into the stand alone headphone amp and I was disappointed. I tried my Sennheiser 6xx headphones which are 200 ohm (I know now, hard to drive) and no good. My Audeze LCD-2 were bad too. Only a pair of portable headphones sounded decent. So after some trial and error I finally hooked my cd player into my Yamaha amp and the Yamaha main rca 100wpc output into the headphone amplifier and it was very good and loud for everything but the Sennheiser 6xx’s which sounded good but not loud. I could use the Yamaha amp volume control and the headphone amp volume control.
Am I doing this right? How does everyone use their headphone amp with their sources? The way I hooked it up I would have to have multiple input/output options to hookup my audio gear so I wouldn’t have to unplug/plugin different sources when I want to hear the speakers with cd then my headphone amp with cd. I don’t think that would work with my Pass Labs X1 and Bryston 4b SST.
Try connecting the Headphone amp's input to the fixed level Tape out of the Yamaha and it should sound better.   You don't want two volume controls in the circuit if you can avoid it
Sorry for asking the obvious, but have you tried the different gain settings on the headphone amp?  With 6 watts of power, that amp should be able to drive all the headphones you mentioned to very loud levels.  Does the CD player have a volume control on it (some do)?

What sources do you want to listen to?  You can connect two devices to that amp and connect the SE pass to your Yamaha. 

As mentioned above, if there's a tape out/record out on your Yamaha, that is probably your best way to connect.
Thanks for the replies, I really appreciate your help in this.

No tape out/record out. I guess it is because it is an older Yamaha made for surround. It has a main out and that is as close to tape out as I will get. I have a tape out on my Pass Labs X1 but I don't have it hooked up. 
My main concern is just using the amplifier with a source it plays my LCD-2's and Sennheiser 6x's on low to medium volume. I understand the 6x's because they are 200 ohm, but the LCD's I felt should play louder. I can't be the only one who listens to music at a high volume level. I'm not talking hearing damage level, just loud. My Sennheiser PX 100's play loud but they are a portable that play loud on an iPhone. It may be my choice of headphones just don't get loud. I'm getting a pair of  Audio Technica ATH-M50's soon so I will probably know then if my decision to get a headphone amp needed the right headphones for my taste or if they just don't make them to listen to loud music.
Yea. Not sure what's going on there. As noted above, that amp should drive your models fine coming direct from the player (the preferred method of the two).