Yes, I understood that you already own the HK receiver. Unfortunately you cannot build upon that receiver - you'll have to replace it before you can improve your amplification.
With the HK at 30 Watts, depending on the size of your room, it is possible that you are running into headroom limits, and I think the Marantz would give you the upgrade capability but probably eliminate the need for it.
Your receiver would no longer be the weak link in your system.
However, you would be wise to check the factors addressed by T2T. The fact that you do get clear sound from your system
sometimes gives credence to his suggestion that it is not your gear.
I just looked up your speakers and at 90db, they are pretty efficient for bookshelfs. However, most bookshelfs are not made to fill a large room, so TLS Guy may be right that the problem is with your speakers.
What are the dimensions of your room? LxWxH
Do you really like to crank it?
Is your system a 2.1? 5.1? or otherwise?
If I had your system and was looking to upgrade
on a budget, I would get the Marantz first, then move the Betas to the rear and get Infiniti P362's for mains (I'm guessing you have a 2.1 system). The P362's are a sweet spot (sound vs cost). However, you would still run into crappy sound out of CD's which are simply not recorded well enough to sound good on a quality system.
One more thing to check. Do you have your Betas set to small in your receiver? I don't know how your bass management works but you need to be sure that you are letting the sub handle the low end and that your Beta's and receiver are freed from having to produce the demanding low frequencies. That could really suck the power out of your receiver in a hurry!