So could HiRez downloads sound as good as SACD?

In short, yes. Yes HiRez can match SACD sound quality and yes you're wrong thinking that the "lossless stuff" in only for redbook CD.
Any computer with USB and a (preferably asynchronous and capable of more than 16/44) DAC will give you hirez audio playback.
Asynchronous USB is the best way to go computer-based, but it's not the only way.
New universal players, like OPPO BDP-95, can play hi-rez files directly from a hard drive, no computer needed. Since it also plays SACD, it will give you a good way to compare the SACD and hirez formats alone while keeping everything else the same.

Thanks for the lesson Mihaitaa!

High res digital has a few different word lengths and sampling rates. Cd is 16/44.1. High res can be 24/88.2, 24/176.4, 24/96, 24/192.

You need a DAC that can convert the different word length/sample rates to analog. There are many DACs available that can do this.

After that the choices and complexity or simplicity of the system grow. There are many different ways to store the music and get it to your DAC. I would do quite a bit of reading before you try putting a system together.

it is possible that for a particular digital front end redbook sounds better than sacd using another particular front end.

i have hirez dvrs, from refereence recordings, and i prefer their sound to sacd . but maybe i don't Have the best sacd player.

so it seems that the answer is a combination of software and hardware

I have modded a number of SACD players over the last 10 years, so I have heard the best that they can do. I dont mod anymore.

Now I use exclusively computer audio interface that can do up to 24/192. I prefer the computer audio interface because I found that SACD often causes overblown dynamics, affecting liveness. Often the CD track on a dual-track CD was more live than the SACD track, particularly on orchestral recordings.

YMMV.

Steve N.
Empirical Audio

I did a little format shootout using high rez files from the 2L label. They sell SACD / BR Audio 2 disc packages and have the same material downloadable in 96/24 and 192/24. So using the exact same source material, I played:

96/24 flac into bridge / perfectwave DAC
192/24 flac into bridge / perfectwave DAC
SACD on Marantz ud9004 universal player
High rez 2 channel PCM on the Blu Ray disc (not sure if it is 192/24 or 96/24)

Although the Perfectwave DAC is a better DAC than the ud9004, I consistently prefered both SACD and PCM on the ud9004 over the flacs. SACD and PCM was a toss up. However, the differences are all fairly small.

To me, the quantum leap comes from going multi channel. No comparison with 2 channel, whatever resolution. Exact same content, I prefer the MCH SACD over the Blu Ray.

The format that gets me closest to the live experience is MCH SACD. Try Chick Corea's RendezVous in New York MCH SACD. To die for.

Edorr - you are comparing apples to oranges. Your SACD player versus the PW DAC. Not the same as comparing the two formats.

If your SACD player had a DAC in it with external inputs that could be driven from a good USB converter, this would be a better comparison, but still not quite apples to apples.

A better comparison would be a 44.1 track ripped from a CD and played on the PW DAC compared to the CD played back from the transport feeding the PW DAC.

Steve N.
Empirical Audio

Steve, I agree. There are a lot of variables here making true apples to apples difficult. The closest I can think of is to get the Ayre Universal which plays all formats on silver discs AND had an asynchronous USB input, and then do the expirement I did. This would take hardware differences out of the equation. All I can say is this. To me MCH SACD is closest to the musical truth I have expienced. In fact, I am listening to Michel Camillo SACD "live at the blue note" as I am typing this up. Sheer delight.

Differences between high rez formats are small, and still tend to favor