How can I start to change the system?

Do the room first if you haven't already. What you'd spend many thousands on in components can probably be done for hundreds if you haven't done room treatments yet.
You can mak the system anyway you want with reflective / absorbtive panels and bass traps.

There are two sets of opinions about building a system: start with the source components, or start with the speakers.

And then there's the third set of opinion...

The Psychic says:

Start with power delivery/noise control. The Archives have plenty of information, there's also a power forum in Audio Asylum sponsored by PS Audio ( biased towards their products, of course).

If you would like to find out if what I'm saying is correct, start in this order with some cheap alternatives that will blow you away:

1) Dedicated line for your system.

2) Cryogenically treated power outlet (I use ACME silver plated cryo).

3)Separate power filtration for digital DVD/DAC. Something simple, like a used ONEAC three ampere isolation transformer/filter from e-Bay (around $25 dollars, 15 pounds). Shipping will be outrageous, so surface mail will be necessay.

4) A good power cord for your Rotel, like those from Ven Haus Audio ( Flavor 4 I recommend).

5) Parallel filtration for your entire system.

"Más vale maña que caña."
--Ismael Rivera

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Thanks for your answers. Now I have a better idea about what I can do.

Sdcampbell: My English teacher wouldn't sign your opinion. I just noticed some big mistakes in my first post. :)

Is really important a ground connection?

Psychicanimal, please could you explain what is exactly "5) Parallel filtration for your entire system"
Thanks
ALAN

Osquitar,
I’m not sure if your original question was answered. If you purchase a tube amp you need a preamp to control the volume, unless your Integrated Amp (Rotel RA01) has the ability to be used as a preamp (it would have connectors (pins) on the back that would let you separate the amp from preamp) and run either amp or preamp separately of each other.
I do agree with Psychicanimal, the power delivery system is more important than most people realize and should be addressed.